What constitutes a rivalry? Is it the passion? Is it the proximity? Is it the familiarity that breeds contempt? Is it the jealousy that one can sense when their rival is racking up the accolades? Is it the infighting between rival fans that leave many hurt, injured or even killed?
Or is it the great play in these matches that bring out the best in both sides, no matter where they are in the league table?
We at Bleacher Report are here to assure you that it's a combination of these factors as we give you the 50 greatest rivalry matches in world football from both club and country.
Sure, they may divide families, friends, cities and countries. But in the end, these derbies are part of what makes the world game the most celebrated sport on the planet.
The Tehran Derby pits Iran's two most popular clubs in Esteghlal F.C. and Persepolis F.C. Persepolis has been seen as a club of the working class, while Esteghlal was seen as a club of the Shah of Iran.
Most matches take place in one of the largest stadiums in Asia, the Azadi Stadium in Tehran, and fan hooliganism has taken place between both sides.
Currently in all competitions and friendlies, Esteghlal lead the series 22-17-31.
The Cairo Derby features two of Egypt's most celebrated clubs: Al-Ahly and Zamalek. Both clubs are among Africa's best sides.
Al-Ahly and Zamalek clash frequently during league, cup and CAF Champions League matches. In addition, both teams are primary feeder teams for the Egyptian national side.
Numerous fights, injuries and deaths have taken place before, during and after the match between fans of both teams, even during the authoritarian regime of Hosni Mubarak.
Thessaloniki's signature rivalry not only extends to the football pitch, but also to the hardwood.
Every year, both teams battle for the most successful club in northern Greece, which has resulted in numerous episodes of fan violence.
Rooted in a rivalry between Greeks and Greek refugees from Anatolia who settled in Macedonia after the population exchange between Greece and Turkey in 1923, PAOK leads the all-time series 48-36-41.
At the Mohamad V. Stadium in Casablanca, two of Morocco's most successful clubs in Raja and Wydad clash in a derby that was first held in 1956.
Wydad fans are considered to be middle and upper class, while Raja are generally associated with the working class.
Wydad have 11 titles, while Raja claim three CAF Champions League crowns and a 37-29-53 record.
Serbia has one of the fiercest rivalries in the Balkans in Partizan vs Red Star Belgrade. Known as the Eternal Derby, this rivalry—which dates back to 1947—spans basketball, volleyball and handball, as well as football.
In spite of the quality of football going down in recent years, the rivalry has been accentuated by increased acts of hooliganism between fans of Red Star (Delije [Heroes]) and Partizan (Grobari [Gravediggers]).
Red Star leads the all-time series 99-71-56.
While the preeminent rivalry in the Netherlands is between Ajax and Feyenoord, the creation of the Eredivisie in 1956 has made the rivalry between Ajax and PSV just as strong. FIFA.com went so far as to refer to it as the "New Klassieker of the Netherlands."
"Beating Feyenoord is a question of honour but beating PSV is about demonstrating our sporting superiority over the rest of the championship," said an Amsterdam ArenA staff member to FIFA.com. "Losing to them leaves a feeling of humiliation."
This is Uruguay's most prominent club rivalry, the Montevideo derby between Nacional and Peñarol. It's also one of the oldest, first contested in 1900.
Peñarol won the Uruguayan Championship on 47 occasions, while Nacional have win the title 41 times.
Peñarol have won 5 Copas Libertadores, 3 Intercontinental Cups and 1 Super Copa (1969), and Nacional have won 3 Copas Libertadores, 3 Intercontinental Cups, 2 Interamerican Cups and 1 Recopa.
As of October 2010, Peñarol leads the series 180-162-159.
This is a fierce dandy of a contest. Croatia's Eternal Derby between Hajduk Split and Dinamo Zagreb dates back to the 1920s, when Hajduk clashed with its predecessor, Građanski Zagreb.
"I've seen a few things in Brazil but when I see the show put on by our fans here," Croatian striker Eduardo said to FIFA.com, "a shiver goes down my spine. It's just gigantic!"
Dinamo and Hajduk remain Croatia's two most popular clubs and since 1946, Dinamo leads the overall series with a 74-64-46 record.
In the Bulgarian capital of Sofia, another Eternal Derby unfolds at the Vasil Levski National Stadium. Gee, this is sounding like a broken record. Every match might as well be an Eternal Derby, no matter what the locale! The rivalry between CSKA Sofia and Levski Sofia first began in 1948.
Both CSKA and Levski two of the most storied teams in the country. Levski claims 31 league titles and 19 cups, while CSKA has won 26 league and cup crowns each.
The success of both teams was so great that in 1985, after a series of scuffles during a cup final, the Central Committee of the Bulgarian Communist Party had to intervene.
Currently, Levski leads the all-time series 77-51-45.
The Avellaneda derby between Independiente and Racing first began in 1907. Another intriguing tidbit: both stadiums are a separated by only less than half a kilometer.
The violence that came from this derby was highlighted in 2006 by hooliganism amongst Racing fans as well as in 1961 by fighting between players from both sides.
Historically, this has been a fiercely contested rivalry, although it pales in comparison to more established rivalries on this list.
Sampdoria, who are staring down the face of relegation as of right now, lead the all-time series 29-21-32.
Like a number of rivalries on this list, Atletico Madrid vs. Real Madrid represent a battle between the aristocrats and the proletariat. In this case, Real Madrid are the team of the aristocrats while Atletico are the team of the working class.
Indeed, the Santiago Bernabeu is located near banks and businesses, while Vicente Calderon is found next to a local brewery.
While Real lead the series by a commanding 80-35-31, the rivalry is nothing short of entertaining and spirited to watch when you are in Spain's capital.
Istanbul clubs Fenerbahce and Galatasaray form Turkey's most storied rivalry, first contested in January 1909. Fenerbahce was founded on the Asian side of Istanbul, while Galatasary were founded on the European side of the city.
As the two most popular clubs, it is not uncommon to see acts of hooliganism between fans of both sides.
Fenerbahce lead the all-time series 140-116-111.
Neighboring countries like to get at each other with a passion. In the case of France vs. Spain, that is a pretty bold understatement.
In Spain's 30 meetings with Les Bleus, La Roja leads the all-time series with a 13-11-6 record. Their first meeting came on April 30, 1922, when Spain blew out the French 4-0.
The North London Derby, which first took place in 1919, is always a heated affair, as the Spurs and Gunners are neighboring clubs.
Few players on either sides are brave enough to play for both teams. In 2001, Sol Campbell's move from Tottenham to Arsenal came as a surprise to Spurs supporters, resulting in their constant derision of Campbell during matches in which he is features against the Spurs.
Arsenal leads the all-time series 71-51-47.
The Derby de Lisboa is always a fun affair, and is another rich vs. poor clash, with Sporting representing the rich and Benfica representing the working class.
Benfica lead the all-time series, winning 68 matches, while Sporting have 45 wins to their credit.
Whenever Club America and Chivas de Guadalajara mix in up in the Primera Division de Mexico, you have yourself one of North America's hotly-contested rivalries: El Super Clasico.
First contested in 1943, America vs. Guadalajara features two of the country's most accomplished teams. America has 10 league titles to their credit which Chivas have 11.
It's a battle between city vs. country: America representing Mexico City, Chivas representing the rest of Mexico. As of right now, it's the city taking command in the all-time standings. America leads the head-to-head with a record of 73-69-63.
Another exciting regional clash involves Spain against Portugal. These two sides first clashed on Dec 18, 1921, when Spain emerged victorious to the score of 3-1.
While Spain dominate this series, it's by the record of 16-6-12, so you can always anticipate an exciting, tenacious affair whenever these Iberian neighbors go after each other.
But in spite of the conduct of the players and the cards shown by the referees, Evertonians and Liverpudlians have been seen sitting side by side.
There is no fan segregation, and no geographical, political, social or religious divides play a key role in this rivalry.
Still, this rivalry on the field is always exciting to watch. Liverpool leads the Merseyside Derby with a 85-66-64 record.
It's a battle of the Olympiques in the Choc des Olympiques between Marseille and Lyon.
The competitive spirit of this showdown, which first began in September 1945, fuels the desire of both teams to best each other.
Marseille and Lyon (as well as Saint-Etienne) are the only teams to have won Ligue 1 four straight times.
There has always been enmity between Japan and Korea, and the enmity goes back centuries.
On the football pitch, both countries have six Asian titles to their credit, and the matches between these two footballing powerhouses are always intense.
Alberto Zaccheroni, the current manager of the Japanese national team, was on record to say that the rivalry matches Brazil vs. Argentina or Italy vs. Germany.
De Klassieker. Plain and simple. The rivalry between Ajax of Amsterdam and Feyenoord of Rotterdam is perhaps the fiercest footballing rivalry in the Netherlands, with fans of both teams clashing against each other.
In fact, the hooliganism was so severe that in February 2009, both cities decided to ban visiting fans from away matches for the next five seasons to curb fan violence.
Ajax leads the all-time series 73-53-38.
Another fun encounter in Europe features Gli Azzurri and Die Mannschaft. Two sides that have seven FIFA World Cups between them? Hard to pass up.
According to FIFA.com, Italy leads the all-time series 14-7-8.
The Manchester derby, which goes back to November 1881, has seen some periods where both teams were unable to face each other on a consistent basis.
Since 2000, however, both teams have dueled in out in the Premier League, and the Manchester derby reached its highest point with City's 1-0 upset of United in the 2010-11 FA Cup semifinals at Wembley.
It's not just the Falklands dispute that fuels England vs. Argentina. A certain "Hand of God" also adds to the storied legacy of this intercontinental showdown.
Ironically, it was the English who introduced the game of football, back in the late 1860's. The first seeds of the rivalry came in 1966, when Argentina's captain Antonio Rattin was sent off for dubious reasons.
Alf Ramsey later referred to the the Argentines as "animals," triggering a caricature of World Cup Willie in pirate garb from the Argentine press.
Then there was Diego Maradona's Hand of God, which you all know was a turning point in the 1986 FIFA World Cup.
While their last match took place in 2005, the battles between England and Argentina have been highly competitive and earn a place in this list. England lead the all-time series 6-2-6.
The Classico Majestoso, which goes back to May 1930, is São Paulo state's showcase rivalry, with the biggest turnouts and many scuffles and skirmishes in between.
Corinthians lead the all-time series 111-89-92.
O Classico. All you need to know. Okay, maybe a little bit more.
This rivalry is geographic, with Benfica representing the south and Porto representing the north of Portugal. And both teams have garnered plenty of hardware.
Benfica own 66 national championships (including 32 league crowns and 24 Portuguese Cups), while FC Porto own 25 league crowns, 15 Portuguese Cups, two Intercontinental Cups, a UEFA Champions League title, a UEFA Cup and European Supercup.
Overall, Benfica has 69 titles while Porto has 67, so you know that when these two get together, you are seeing the two best sides in all of Portugal.
Porto lead the all-time series 85-82-53.
It's the Old Firm! A rivalry that goes back to the sectarian divides between Catholics and Protestants in Scotland. Rangers for the Protestants, Celtic for the Catholics.
While this is no longer the case regarding religion, both Celtic and Rangers are the two most accomplished Scottish club sides in history. According to the BBC, Rangers vs. Celtic brings more than £120 million to Scotland's economy.
This rivalry, which dates back to May 1888, features two teams that have won 95 Scottish League crowns, 67 Scottish Cups and 41 Scottish League Cups.
Whether you're at Ibrox Stadium or Celtic Park, you know you're in for a special one when these two sides tangle it out.
Rangers lead the all-time series 157-143-95.
The rivalry between Mexico and the United States in football used to be a one-sided affair.
However, in the mid-1990's, the Stars and Stripes have been getting the best of their neighbors south of the border, and now the caliber of play between both teams is becoming more evenly matched.
Mexico lead the all-time series 32-15-11, but with the USA improving as a footballing nation, look for the gap to close slowly.
Still, in spite of differing priorities, any match involving the Blues and the Gunners is a treat to watch, whether it's an FA Cup Final, Football League Cup Final, a match in the UEFA Champions League knockout stage or just a Premier League tie.
Arsenal lead the all-time series 69-54-51.
Separated by only the English Channel, England and France have a deep, intense hatred for one another. A battle between the Three Lions and Les Bleus is also a battle between rosbifs and frogs.
Hungry, anybody? Of course, this is also a rivalry fit for the rugby pitch, but that is a different story.
England lead the all-time series 16-8-4.
Greece's most stories rivalry comes, naturally, from the capital city.
Once again, this is another rich vs. poor showdown. Panthinaikos representing the wealthy, Olympiacos representing the working class. Well, it was a rich vs. poor showdown. Now all social classes are represented in this one.
Olympiacos have won 37 Greek League crowns, 24 Greek Cups and three Greek Super Cups, while Panathinaikos claim 20 Greek League titles, 17 Greek Cups and three Greek Super Cups.
And there's the customary football hooliganism going around from fans of both teams. In fact, it's so rife that people get killed.
Since 1959, Olympiacos leads the all-time series 57-35-53.
We love high-profile matches between European countries because they are among the best at the world game. When France and Italy meet on the pitch, you are guaranteed some of the finest football between two geographical rivals.
The high point of this rivalry came in 2006, when Italy and France played to a 1-1 draw that went all the way to penalties. Italy went on to win their fourth star, 5-3.
The Azzuri lead the all-time series 17-7-10.
England vs. Scotland is always a hotly contest match between the two best sides among the Home Countries.
This is the oldest international fixture in the world, first contested in 1872. In the past, England and Scotland both clashed on a regular basis, but rivalries with Argentina and Germany have diminished this rivalry somewhat.
Nonetheless, England leads the all-time series by a razor-thin 45-41-25.
This is one of the newer rivalries that really got started recently, back in 1974, when the Netherlands lost to then-West Germany 1-0 in the FIFA World Cup.
However, the anti-German sentiments from World War II fueled the fires, and it was not until 1988 that the Oranje would get their revenge against Die Mannschaft to win the European Championship.
Today, the sentiments of war are not as strong, allowing the game of the pitch to be the true battle for both sides. Germany lead the all-time series 13-10-14.
Whether at Old Trafford or Stamford Bridge, any contest involving the Red Devils and the Blues is sure to entertain.
This is one of the newer rivalries in the Premier League that has gained some steam. Even though Arsenal and Liverpool are considered to be Manchester United's main opponents, the rise of Chelsea as a competitive threat has made the enmity between the Red Devils and Blues a bit more stronger.
One of the high points of this rivalry came in 2007 at the new Wembley Stadium, when Chelsea, under then-manager Jose Mourinho, defeated Sir Alex Ferguson's United squad 1-0.
This is Le Classique, the marquee showdown between France's top two club sides: Paris Saint-Germain and Olympique de Marseille.
Like Manchester United vs. Chelsea, this is one of the newer rivalries of football, which began in December 1971. The rivalry reached its peak during the 1980's and 1990's, and the crowds pack the Parc des Princes and Stade Velodrome to the rafters.
Between them, both teams have garnered 11 Ligue 1 titles, 18 French Cups, four League Cups, four Champions Trophies, a UEFA Cup Winners' Cup and a UEFA Champions League title.
Hooliganism has intensified the rivalry, with numerous incidents between rival fans and police in the past two decades.
Marseilles lead the series 31-26-19.
Here's another exciting showdown you have to see for yourself: Lazio vs. Roma, the Derby della Capitale.
First staged in December 1929, this rivalry features some great football to go with massive turnouts, violence, racist banners, and a level of passion from both sides that is rivaled by few.
One of the boiling points of this rivalry came in 2004, when rumours of a young boy killed by police began to percolate across the stadium. The captain of Roma at the time, Francesco Totti, asked for the match to be called off.
When the match was postponed, a riot occured between fans and police to go with streets lit on fire. Worthy of a cheesy rock music video, one would imagine.
Both Flamengo and Fluminense have plenty of accolades to show for it. Flamengo have won 31 state titles, six Serie A crowns, two Copas do Brasil, one Copa Libertadores and one Intercontinental Cup.
Fluminense have won 30 state titles, three Serie A crowns, one Copa do Brasil and a Taça Rio.
This rivalry goes all the way back to July 1912. Currently, Flamengo leads the all-time series 135-119-122.
Two footballing powerhouses. Two proud national sides. One fitting rivalry. That is England vs. Germany, the Three Lions vs. Die Mannschaft.
First contested at the turn of the 19th Century, England and Germany's rivalry hit one of its highest points in the 1966 FIFA World Cup Final, when a questionable goal from Geoff Hurst in extra time was ruled in, even though visual evidence suggested otherwise.
In addition, a fourth goal saw its home fans flood the pitch.
Even though Germany have dominated the rivalry in recent years, England still lead the all-time series 12-11-5.
Who could forget the Superclasico, Boca vs. River? Rivalries with as much magnitude and attentional globally as Boca Juniors vs. River Plate will always show up on any list of the top rivalries in World Football, and it's no surprise that it's in this one as well.
Whether it's at La Bombonera or El Monumental, this clash, which goes back to August 1913, divides families, friends, the city of Buenos Aires and Argentina.
More than 70 percent of Argentina's fans are either Boca or River supporters, and Gavin Hamilton of the Observer noted that watching a match like this is one of the 50 things one must do before he or she goes to the man upstairs.
Boca Juniors leads the overall series with a 121-105-102 record.
The San Siro in Milan is home to one of the best club football rivalries in Southern Europe, if not the world.
Internazionale and AC Milan do not like each other at all, and when both teams get on the pitch, fans can expect intense play and even more intense partisanship.
The Derby della Madonnina goes back to January 1910, and also has socio-economic roots, with Milan seen as the club for the working class and Inter for the club of the wealthy.
Inter have garnered more domestic championships (29), while AC Milan have earned 14 continental titles and four intercontinental crowns. Overall, both Milanese sides have 83 championships between them.
AC Milan leads the overall series 107-95-72.
South America's most storied international rivalry, the Battle of the South Americans is sixth on this list.
The rivalry between Brazil and Argentina goes back centuries, during the colonization of the Americas.
Both Brazil and Argentina claim seven FIFA World Cups, 22 Copa America, four Confederations Cups, two Olympic Gold Medals, 10 Pan American Games crowns, 10 FIDA U-20 World Cups and three U-17 World Cups.
This matchup has had the privilege of featuring a unique player rivalry in itself. Diego Maradona vs. Pele is akin to that of Larry Bird vs. Magic Johnson in terms of talent, accolades, popularity and so forth.
Argentina leads this hotly contested series 34-33-23.
The Derby d'Italia makes the top five of this list of rivalry matches, as it features two Italian clubs with significant success internationally and domestically.
Both Juventus and Inter and ranked first and second in wins and goals in Serie A. This rivalry goes all the way back to Nov. 14, 1909.
Juventus leads the all-time series 94-67-53.
Another matchup featuring two teams with numerous accomplishments between them is Manchester United vs. Liverpool. The North West Derby goes back to April 1894, and is generally played during the midday thanks to TV interests and the safety of the fans.
In an interview with the BBC, Manchester United's Ryan Giggs was quoted as saying that this rivalry is "probably the most famous fixture in English football."
Both Manchester United and Liverpool have won nearly 60 trophies each, although this could change for the former within the next few weeks.
Manchester United lead the all-time series 71-61-50.
But while Manchester United and Liverpool have a strong rivalry in their own right, the enmity between Arsenal and Man U is even stronger.
The reason? Longtime managers Arsene Wenger of Arsenal and Sir Alex Ferguson of Manchester United.
Any time both Arsenal and Manchester United take to the pitch, it's a battle of wits between both managers, who are arguably the best in the Premier League and all of England and possibly the world, although the latter is certainly up for debate.
So far, Fergie has the edge, with a 20-16-9 record.
El Clasico. Need we say more?
Now FC Barcelona vs. Real Madrid is not ranked this high solely because of the recent set of encounters between Los Blancos and the Blaugrana in the past few weeks.
It's because of that and the other meetings between these Real Madrid and Barcelona over the course of decades that make this the top-ranked rivalry on this list.
This rivalry, which goes back to May 13, 1902, is watched by the masses around the world, live and on the tube. The wealth and domestic and international accolades that both teams possess make any match of El Clasico worth replaying over and over again.
In spite of FC Barcelona taking the overall series for the 2010-11 season, Real Madrid still leads the overall series with a record of 85-82-43.
Ladies and gentlemen, readers...this is the rivalry to end all rivalries. It's Brazil vs. Italy.
We're talking about two of the most accomplished countries in world football. Brazil have garnered five FIFA World Cups, Italy four. That's nearly half of the FIFA World Cups in history. So any time these two tangle together, be it friendly or a tournament, it's never a dull moment.
Currently, Brazil lead the all-time series 7-5-2.