In Phil Ball's book on Spanish football, he says that, between Barcelona and Real Madrid, there is "so much morbo you would have to employ a very powerful priest to exorcise the phenomenon."
However, he can't quite put his finger on how to describe what is known in Spain as "morbo"—though you can probably imagine it has a fair amount to do with the intensity that surrounds the rivalry.
With that intensity in mind, it's not hard to understand why there have been so many thunderous editions of El Clasico through the years.
And, rather than improving over time, the relationship between the two clubs has worsened, as they have become more dominant than ever in the 21st century—leading to more frequent and more fierce versions of El Clasico.
Rewinding back over the years, though, here are 10 of the greatest moments from one of the world's greatest fixtures.
In Jose Mourinho's first season as Real Madrid boss, he led the club to a final showdown with Barcelona in the Copa del Rey.
It was the first time the two sides had met in the final stage since 1990, and just the fifth time since 1950.
Barcelona had not just been the top team in Europe for the past few years, but had also enjoyed a significant advantage over Madrid too—Mourinho lost 5-0 in his first match against them that season.
The match itself was a nervy affair, with the two teams remaining goalless after 90 minutes.
Cristiano Ronaldo stepped up to the plate in extra-time though, scoring the winner and providing Los Blancos with their only piece of silverware that season.
It's just a shame Sergio Ramos dropped the trophy on the open top bus parade!
Not a particularly entertaining game—it ended 0-0—but Real Madrid's visit to Barcelona in 2002 has become one of the most remembered meetings of all time.
It became known as the "Derbi de la Verguenza"—or "Derby of Shame"—and centered around a certain Luis Figo.
Figo had upset Barca's fans by joining the Galactico ranks at the Bernabeu two years earlier but, for some reason, they were particularly angry on this occasion.
The Portuguese international, who was taking corners, had all number of objects launched at him from Camp Nou's terraces—including, famously, the head of a suckling pig and a full bottle of whiskey.
Roberto Carlos led the Madrid players off the pitch in protest and the match was held up for 10 minutes.
When it resumed to conclusion, the excitement on the pitch couldn't match the feverish atmosphere in the stands.
The 1993/94 season may have ended on a sour note for Barcelona's Dream Team, dismantled by AC Milan in Europe, but they'll always have this Clasico humiliation.
Brazilian goal scoring machine Romario was on target three times as La Blaugrana roughed Real Madrid up inside Camp Nou.
The match finished 5-0, with the other goals coming from a trademark Ronald Koeman free kick and substitute Ivan Iglesias.
It wasn't just to prove Barca's last season as Spain's No. 1 side, though, but their last season with Michael Laudrup too.
In the summer following the 5-0 win against Real Madrid, Michael Laudrup took the Luis Figo route—before Figo himself had taken it—and controversially departed for the Spanish capital.
It signaled a swing in La Liga, as Los Blancos would go onto clinch the title once again.
En route to the championship, Madrid exacted revenge on Barca for the previous season's 5-0 defeat at Camp Nou—they won 5-0 themselves at the Bernabeu.
Chilean Ivan Zamarano was the man to steal the headlines on this occasion, though, not Laudrup, as he helped himself to a hat-trick.
Luis Enrique—who would later swap Madrid for Barca, himself—scored too, as did Jose Emilio Amavisca.
The 2011 Champions League semifinal between Real Madrid and Barcelona was squashed in around the Copa del Rey final and a Liga Clasico.
It was an extremely intense time in the rivalry, with Pep Guardiola and Jose Mourinho playing central roles in the four fixtures.
Two late goals from the brilliant Lionel Messi at the Bernabeu, in hindsight, put Barca into the final before Madrid even departed for Catalunya.
In the return leg, Marcelo did briefly level the match after Pedro Rodriguez had given La Blaugrana the lead, but Barcelona never looked like relinquishing their lead.
Barca went on to beat Manchester United in the final, while Los Blancos were left still dreaming of La Decima.
Real Madrid were the undisputed kings of Europe—no other side had won the competition since it had started—when they drew Barcelona in the semifinal in 1960.
The first leg was played at the Bernabeu in front of over 100,000 fans and it was won by the home side.
A brace from Alfredo Di Stefano, and one from Ferenc Puskas helped Los Blancos to a 3-1 win ahead of the return leg in Catalunya.
Unfortunately for Barcelona it was the same story at Camp Nou, as Madrid's impressive European form continued.
Puskas was the man to score two this time, while Francisco Gento added the third in another 3-1 win, as Madrid progressed to the final 6-2 on aggregate.
Once in the final, they took apart Eintracht Frankfurt 7-3.
You know you've witnessed something special when the Real Madrid fans can cast aside any feelings of morbo to give a Barcelona player a standing ovation inside the Bernabeu.
That was what happened when Ronaldinho picked apart Los Blancos in 2005.
Samuel Eto'o gave Barca the lead in the first half, but it was the Brazilian who took over after the break.
His skills were sublime. They weren't just for show either, they came with plenty of substance.
When he picked the ball up on the half way line in the second half, Madrid appeared in no particular danger, but he danced into the area before smashing the ball home.
Then he added a third with a second goal which wasn't bad either (03:10).
Real Madrid got the better of their old enemy when the two sides met in the semifinal of the Champions League in 2002.
The first leg was played at Camp Nou, and Los Blancos silenced the home crowd with a 2-0 win.
Zinedine Zidane gave Vicente del Bosque's side a second half lead, but it was Steve McManaman who did the damage when he added a second in stoppage time.
Back at the Bernabeu, this rocket from Raul all but put the tie out of Barca's reach.
An Ivan Helguera own goal did briefly give the visitors some hope, but it was Madrid who eased into the final.
Goals from Raul and, memorably, from Zidane helped beat Bayer Leverkusen to lift the trophy.
Alfredo Di Stefano had already played a couple of friendlies for Barcelona, who had moved to sign him having been impressed with his performances in Spain on tour with Colombian club Millonarios in 1952, when Real Madrid got involved.
It all got very murky from there.
Nobody was sure who was due money for Di Stefano's signature. Madrid had paid Millonarios 1.5 million pesetas for him, but Barca had paid four million to River Plate.
Among the madness the Spanish federation banned the signing of foreign players, before lifting it again. Out of tiredness, and having grown fed up with the Madrid-based government getting involved, Barcelona withdrew their attempts to sign the Argentinian.
Real paid the Catalans the four million they had paid to River Plate, and Di Stefano joined Los Blancos.
Two weeks later he made his controversial El Clasico debut: Real won 5-0, and he scored four.
The rest, as they say, is history.
As Barcelona stalled over the signing of Ajax's Johan Cruyff, Real Madrid, in a saga not overly dissimilar to the Alfredo Di Stefano one, decided to get involved.
This time the outcome was different though, Barca quickly stumped up the cash to make sure they landed the Holland international.
Cruyff told the European press, upon his arrival in 1973, that he had chosen Barcelona because he could "never play for a team associated with Franco," via Phil Ball's Morbo.
Already adorned to the Catalans after that comment, the levels with which they worshiped Cruyff would know no bounds after his first visit to the Bernabeu as a player.
He set up three and scored one as Barcelona recorded a 5-0 win in the 1973/74 season en route to their first La Liga title since 1960.