The Champions League is the premier competition in Europe, pitting all of the continents top teams from its various leagues against one another annually, with the chance to earn the distinction of being the best club in the world.
But sometimes, it's about far more than simply being named the top squad around. Sometimes—and especially for those teams struggling in their domestic campaigns—it's a chance to earn some much-needed plaudits in an otherwise poor season.
For the truly elite clubs in world football, a season without some form of domestic or continental success is simply unacceptable.
Let's break down three teams with a whole lot to lose in the knockout phase of this year's Champions League.
United may need to win this tournament just to return to it next year. And no, that's not being dramatic.
A repeat of last year's Premier League championship seems nearly impossible at this point, with United now 13 points behind league-leading Arsenal.
But worse, United are seven points behind Manchester City for fourth place in the league, where they would have to finish in order to reach this competition next season.
With Arsenal, Chelsea and Man City looking like locks to finish in the top four, United will have to battle past Liverpool, Everton and Tottenham to finish in a Champions League qualification position, no easy task.
And with Robin van Persie out for a month, according to BBC Sport, they could fall even further behind in that race.
Add it all up and the Champions League is not only an opportunity for the Red Devils to salvage some positivity from a season on the rocks, but could also be the only way they qualify for next year's tournament.
Expect them to go into the Round of 16 in full desperation mode unless they could turn their domestic campaign around.
Like Manchester United, Milan may need to win this tournament just to return next season. But for the Italian side, the stakes are even higher.
In Serie A, just three teams qualify for the Champions League, and Milan are already 14 points behind third-place Napoli. Yes, 14 points. Barring a miraculous turn-around, they aren't coming back next season.
Unlike Manchester United, this Milan side doesn't have the horses to win the Champions League (and yes, for United even it's a stretch). But at least getting to the quarterfinals or semifinals and saving some face in an otherwise abysmal season could make a huge difference.
This is obviously a transition period for Milan. A strong showing in the Champions League could facilitate a much faster return to prominence.
Dortmund won the Bundesliga in consecutive seasons between 2010 and 2012. Then, last year, Bayern Munich trumped them in both the league and in the Champions League final. Suffice to say, expectations are sky high for this team.
But this season has been a struggle.
While Dortmund will still likely qualify through their domestic campaign for the Champions League, they survived the group stage by the skin of their teeth—albeit in a Group of Death that included Arsenal and Napoli—and are currently in third place in the Bundesliga table, already 10 points behind Bayern Munich.
It's unlikely they'll catch the Bavarians, meaning the most meaningful title still within their grasp is the Champions League hardware. And assuming Robert Lewandowski doesn't bolt in January, this is the last season the club will likely have the world-class forward.
And teams are going to be knocking down their door trying to lure talented attacker Marco Reus away, too.
Dortmund has a window of opportunity to be Europe's best this season—failing to capitalize, while expectations are extremely high, would be a disappointment for the revitalized club.