All you need to know about this week's first legs in the UEFA Champions League quarter-finals. What's the history between the two teams and who has the edge from past contests? Which sides are looking to overturn the weight of history? Who's performing well on the domestic front? Find out below.
Manchester United FC v FC Porto
• United are unbeaten in 21 matches in the UEFA Champions League and enter this contest also aiming to extend a run of 19 games without defeat at Old Trafford in the competition.
• Porto have never won in England, where ten of eleven visits have ended in defeat, although the one occasion when they did avoid defeat – a 1-1 draw at Old Trafford in the 2003/04 first knockout round second leg – eliminated United from the competition.
• This is United's 15th quarter-final tie in Europe's élite club competition and their record is: P14 W10 L4.
• Sir Alex's men beat AS Roma at this stage in both 2007 and 2008 and are now seeking a third consecutive semi-final appearance – a first for the English champions.
• Porto have won three of their five quarter-finals in Europe's premier club competition and have triumphed in two of the clubs' three previous two-legged contests.
Villarreal CF v Arsenal FC
• This tie is a repeat of the 2005/06 semi-final, won 1-0 by Arsenal thanks to Kolo Touré's first-leg goal in the last European match played at Highbury – although the London club needed a last-minute penalty save by Jens Lehmann from Juan Román Riquelme in the second leg to escape El Madrigal with a goalless draw.
• Villarreal's record against English clubs is: P9 W3 D5 L1. At home their record is: P5 W2 D3 L0. In those five past meetings with English opposition at El Madrigal, they conceded just one goal scored, curiously, by a Spaniard – Everton FC's Mikel Arteta.
• Arsenal have lost three European finals to Spanish opposition. They lost 5-4 on penalties to Valencia CF following a goalless draw in the 1979/80 UEFA Cup Winners' Cup final and succumbed 2-1 to Real Zaragoza in the final of the same competition in 1994/95 before a 2-1 defeat by FC Barcelona in the 2005/06 UEFA Champions League final in Paris.
• That defeat by Barcelona was Robert Pirès' final appearance for Arsenal after six seasons with the club, during which he won two Premier League titles and two FA Cups. Voted the English Football Writers' Player of the Year in 2002, he made 284 appearances, scoring 84 goals, before moving to Villarreal in summer 2006.
• Villarreal have kept only four clean sheets in their last 30 matches in all competitions having managed six in their opening nine games this season. In contrast, Arsenal have kept clean sheets in six of their last eight Premier League games, and have conceded only four league goals in 2009 – all away from home – fewer than any other side.
FC Barcelona v FC Bayern München
• Bayern and Barcelona are the joint-highest scorers in this season's competition with 24 goals each from eight games. In Barcelona's case they have also conceded the most goals of any of the last-eight participants – eleven – and have still to keep a clean sheet on home soil.
• The visitors are looking to improve on a recent record of three consecutive defeats at this stage of the UEFA Champions League – in 2002, 2005 and 2007.
• Bayern have won three of the four previous meetings with Barcelona but the Liga leaders boast an impressive run of form against German opposition having won ten and drawn one of their last eleven matches against sides from the Bundesliga.
• Coaches Josep Guardiola and Jürgen Klinsmann were both in action when Barcelona and Bayern met for the first time in the semi-finals of the 1995/96 UEFA Cup. The teams drew 2-2 in Munich before Bayern – with Klinsmann again present, but Guardiola absent – advanced to the final with a 2-1 success at Camp Nou.
• Thierry Henry's next goal in the UEFA Champions League will be his 50th in the competition.
Liverpool FC v Chelsea FC
• This is the fourth time in five seasons the teams have met in the knockout stages, following semi-final contests in 2005, 2006 and 2008. On each previous occasion the team playing the second leg at home progressed.
• Five-times European champions Liverpool can take heart from home-and-away wins against their opponents in the Premier League this season – not to mention an unbeaten home run in the UEFA Champions League that stretches to ten matches, including qualifiers.
• Both teams have formidable recent records when it comes to surmounting this particular hurdle: Liverpool won quarter-finals in 2005, 2007 and 2008, while Chelsea advanced in 2004, 2005, 2007 and 2008.
• Chelsea have not won any of their last six away matches in the UEFA Champions League, while Liverpool are unbeaten in five home meetings with Chelsea and won the last Anfield encounter 2-0 in February thanks to Fernando Torres goals after 89 and 90 minutes.
• Liverpool have won four of six previous two-legged contests against English sides in UEFA club competition. In addition to their UEFA Champions League successes against Arsenal FC in last season's quarter-finals and those Chelsea ties, they beat Tottenham Hotspur FC on away goals in the 1972/73 UEFA Cup semi-finals following a 2-2 aggregate draw.
