The first leg meeting between Barcelona and Chelsea was something of an outlier in both team's seasons.
Barcelona are unbeaten in the league and have usually played with a consummate level of control but were rattled and a little fortunate as Willian hit both posts before opening the scoring. It was only after exploiting a defensive error that they found themselves back in the game.
Chelsea, on the other hand, have been unable to retain their title, with qualification for next year's Champions League in serious doubt after losing four out of five Premier League games. But in the first leg, they put in an excellent performance as they nullified their opponent's goal threat and created good opportunities.
After that 1-1 draw, the tie hangs in the balance - though Barcelona undoubtedly have the edge after Lionel Messi's away goal. Andres Iniesta could return for Barcelona, which would be a big boost after the injury he suffered against Atletico Madrid - especially as Philippe Coutinho is cup-tied.
Meanwhile, Chelsea have no injury concerns that will affect their first team.
Here are five things to expect: