UEFA.com surveys the form of each round of 16 contender, and tells you a bit about each team too.
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Group winners (seeded): Barcelona (ESP), Bayern München (GER), Borussia Dortmund (GER), Juventus (ITA), Manchester City (ENG), Paris Saint-Germain (FRA), Porto (POR), Real Madrid (ESP)
Group runners-up (unseeded): Ajax (NED), Atlético Madrid (ESP), Liverpool (ENG), Lyon (FRA), Manchester United (ENG), Roma (ITA), Schalke (GER), Tottenham Hotspur (ENG)
Nickname explained: United didn't start out wearing red – they wore blue and white and also green and yellow before turning red when they took on their current name in 1902. 'Red Devils' was initially an affectionate nickname, but gradually became part of the club's identity, a little imp making its way onto the badge in the early 1970s.
Did you know? A St Bernard called Major helped underpin United's success. Club captain Harry Stafford lost the dog in 1901 after it wandered out of a team fundraising event, but Major was found by local pub owner John Henry Davies. Davies and Stafford proceeded to strike up a friendship, and Davies ended up a major investor in United and their first chairman.
Nickname explained: PSG stands for Paris Saint-Germain; the club was formed from the merger of Paris FC and Stade Saint-Germain (from the western suburb of Saint-Germain-en-Laye) in 1970.
Did you know? Prêt-à-porter fashion pioneer Daniel Hechter – who became the club's president in 1973 – designed their shirt to incorporate the red and blue of Paris and the white of Saint-Germain. Wearing this kit, they reached European semi-finals in five successive seasons from 1993–97, lifting the 1995/96 UEFA Cup Winners' Cup.
Nickname explained: Most call Roma the Giallorossi (Yellow-Reds) in honour of their club colours, but some nod to their home city's creation myth by labelling them 'La Lupa' (She-Wolf) – Rome's founders Romulus and Remus having been suckled by a wolf as babies, as depicted on the club's badge.
Did you know? One-club wonder Francesco Totti was not just a hero for Roma fans; a 2011 poll found he was the most popular player in the world. The archetypal No10 represented his home-town team from 1992–2017; his wedding to Ilary Blasi in 2005 was such a big deal in Italy that it was broadcast live on TV.
Nickname explained: The club are dubbed Dragões (Dragons) in reference to the red dragon on their badge, which bears the slogan 'Invicta' (unbeaten) – because the city of Porto is regarded as being invincible.
Did you know? Porto's crest was designed in 1922, a merger of their first badge – a blue football bearing the letters 'FCP' in white – and the city's coat of arms. The graphic artist responsible was Augusto Baptista Ferreira, aka Simplício, who was also a Porto player that scored 23 goals in 26 games.
Tottenham v Dortmund
Nickname explained: Spurs' nickname derives from a notable medieval rebel nobleman, Sir Henry Percy (1364–1403), youngest son of the Duke of Northumberland, whose derring-do earned him the nickname 'Harry Hotspur' – and a key role in Shakespeare's play Henry V.
Did you know? Bad light and a dead parrot helped intensify Tottenham's rivalry with north London neighbours Arsenal. In November 1887, Spurs led 2-1 in their first meeting with the Gunners, only for the game to be abandoned due to bad light with 15 minutes left. Then, Tottenham's pet parrot – gifted by a ship's captain during a 1908 tour of Argentina and Uruguay – died on the day Arsenal were given Spurs' place in an enlarged English top flight in 1919.
Nickname explained: Usually known by their initials, BVB, or their colours, Die Schwarzgelben (Black-Yellows), Dortmund's other nickname – Die Borussen (the Prussians) – is slightly misleading. It relates to a beer once made in a nearby brewery; Prussia is in the north-east of Germany, whereas Dortmund is in the west.
Did you know? In an August 1986 game against Bayern, Dortmund striker Frank Mill had one of the worst debuts ever, conspiring to hit the post despite being under no pressure after rounding keeper Jean-Marie Pfaff. "It was the worst moment of my career," said Mill, now 60. "Six months later I saw a clip on American TV, calling it one of the greatest blunders in European football. It still makes me mad if anyone asks: 'What happened to you in Munich that time.'"
Ajax v Real Madrid
UEFA ranking: 24
Group stage: W3 D3 L0 F11 A5
Top scorer: Dušan Tadić (5)
Last season: third qualifying round (UEFA Europa League play-off round)
European Cup best: winners x4 (1971, 1972, 1973, 1995)
Nickname explained: Ajax are Die Godenzonen (Sons of the Gods), even though their namesake Ajax was in fact the grandson of the god Zeus in Greek mythology.
Did you know? Bob Marley's Three Little Birds is a terrace anthem at the Johan Cruijff ArenaA. It all started in 2008 during a friendly against Cardiff City when stadium announcer and DJ Ali Yassine, a former guitarist in a reggae band, decided to play as much reggae as possible. Ajax fans now proudly belt out the chorus home and away.
Real Madrid (ESP, holders)
UEFA ranking: 1
Group stage: W4 D0 L2 F12 A5
Top scorer: Gareth Bale, Karim Benzema (3)
Last season: winners
European Cup best: winners x 13 (1956, 1957, 1958, 1959, 1960, 1966, 1998, 2000, 2002, 2014, 2016, 2017, 2018)
Nickname explained: Madrid are widely known as Los Merengues (Meringues) because of the perceived similarity between their all-white kit and the colour of the popular confection of the same name.
Did you know? Santiago Bernabéu, who gives his name to the club's stadium, was a player, coach and president of Real Madrid. The venue that carries his name is the only stadium in the world that has held the final of the UEFA European Championship, FIFA World Cup, UEFA Champions League and Copa Libertadores.
Liverpool v Bayern
Nickname explained: Liverpool are the Reds, but they wore blue and white shirts until adopting the colours of their home city in 1894. The decision to add red shorts and socks came in 1964, manager Bill Shankly feeling an all-red outfit was more imposing for opponents, red symbolising danger.
Did you know? Liverpool would never have been formed had the city's original club Everton not left Anfield to move to another stadium, Goodison Park, in 1892. The Reds were then founded by John Houlding to ensure there was a club to play at the empty venue.
Nickname explained: FCB, Die Bayern (Bavarians) and Die Roten (Reds) are easily explained, while the more disparaging moniker 'FC Hollywood' refers to the club's high profile in the 1980s and 1990s – and the tendency for their players to feature as much in newspaper gossip columns as in sports pages.
Did you know? A slap in the face altered the course of Bayern's history. In 1958, 1906 München prospect Franz Beckenbauer was set to join 1860 München – then the city's top side. However, the 13-year-old changed his mind after being slapped in the face by an 1860 defender: he was so enraged he joined Bayern instead, where 'Der Kaiser' became a club great.
Lyon v Barcelona
Nickname explained: Lyon supporters are 'Les Gones', a local word that roughly translates as 'child' or 'son'. However, you do not get to be one just by vaguely liking the club – a 'Gone' is a genuine, fanatical follower.
Did you know? Lyon figure in the biggest rivalry in French football. The balance of power between Lyon and neighbours St-Étienne has shifted since Les Verts' 1970s heyday, but Rhône-Alpes derbies remain massive. Lyon fans never wear green, with one-time OL stalwart Maxime Gonalons labelling Ligue 1 meetings with St-Étienne "the two most important matches of our season, whatever happens".
Nickname explained: Blaugrana (Blue-Clarets) owes to Barcelona's shirt colours. Exactly why they chose them is a mystery: they are either the colours of the French First Republic, the colours of Merchant Taylor's School in Merseyside (attended by the club's first president Arthur Witty) or the colours of FC Basel, founded by Barça pioneer Joan Gamper.
Did you know? Some call Barcelona fans Culés ('buttocks' in Catalan), a nickname that dates back to the early 20th century when supporters were forced to sit on the stadium's perimeter walls since there was not enough room inside. Pedestrians walking by on matchdays could see nothing but fans' bottoms, hence Culés.
Atlético v Juventus
Atlético Madrid (ESP)
UEFA ranking: 4
Group stage: W4 D1 L1 F9 A6
Top scorer: Antoine Griezmann (4)
Last season: group stage (UEFA Europa League winners)
European Cup best: runners-up x3 (1974, 2014, 2016)
Nickname explained: Traditionally, mattresses on Spanish beds were covered with red-and-white striped material, so Atlético with their red-and-white striped shirts came to be known as Los Colchoneros (Mattress Makers).
Did you know? The bear and strawberry tree on Atlético's badge is one of the symbols of Madrid; a statue of the bear and strawberry tree stands at Puerta del Sol, one of the city's main squares and the point from which distances for Spain's main motorways are measured.
Nickname explained: Bianconeri (Black and Whites) is easy enough to grasp; La Vecchia Signora (Old Lady) takes a bit of explaining. Once 'la Fidanzata d'Italia' (Girlfriend of Italy) due to their widespread support, the club's ageing team of the 1930s contrasted starkly with the name Juventus (meaning 'youth'), so they were cheekily dubbed Old Lady – and it stuck.
Did you know? Juve originally wore pink shirts (with black tie!), changing kit in 1903 when those first outfits started to fade. The club sent for replacements from England and got black-and-white shirts, specifically replicas of the tops worn by Notts County.
Schalke v Manchester City
Nickname explained: The Königsblauen (Royal Blues) are also called Knappen (Miners) as a nod to their background; while the club were formed at a local high school, many of their players and supporters were the coal workers of their home city, Gelsenkirchen.
Did you know? Schalke are so proud of their coalmining heritage that in 2014 they redesigned the tunnel leading up to their Arena AufSchalke to make it resemble a mine shaft.
Nickname explained: Originally a church team giving men in the eastern suburb of West Gorton something healthy to do, the club became Manchester City in 1894, hence their functional nicknames – City and Citizens.
Did you know? The fans' song of choice, poignant romantic ballad Blue Moon, evinces a certain embattled optimism, a measure of a club who seemed to thrive on adversity at times; tellingly, they are the only side to be relegated from England's top flight as reigning champions, going down the season after winning their first title in 1936/37.