They are the future -- and in some cases the present -- of football on the European stage. The 21 players listed below, about to embark on 2016-17 seasons in England, Germany, Italy, Spain or Portugal, represent the game's top tier on the continent where the sport's passions run deepest. A look at the top figures in the youth movement, aged 21 or under:
(For a photo gallery including all of the 21 under 21 stars, click HERE).
Dele Alli, Tottenham Hotspur (20 years old)
The attacking midfielder had a sensational debut season for Tottenham, notching 10 goals and 9 assists in 33 league appearances after arriving from League One club MK Dons. Now 20, Alli looks set to continue his blossoming partnership with Harry Kane. Few youngsters offer such an exciting blend of technique and athleticism, and, in Mauricio Pochettino, he seems to have the ideal manager supervising his development.
Hector Bellerin, Arsenal (21)
Another gem from the Barcelona academy, Bellerin has been polished into one of the finest full-backs in the Premier League. The Catalan, who combines blistering pace with the flair of a winger, received more passes in the final third than any other defender last season. That makes him a fine fit for Arsenal's adventurous style, though Arsene Wenger should be aware of future interest from the Camp Nou.
Kingsley Coman, Bayern Munich (20)
The quicksilver winger has proven himself as one of the continent's most exciting attackers while in Bavaria. Only one first-team regular pulled off more dribbles per 90 minutes in the Bundesliga last season. With Arjen Robben and Franck Ribery both injury-prone and well past 30, Bayern will surely choose to buy him permanently before his loan deal from Juventus expires next year.
Ousmane Dembele, Borussia Dortmund (19)
One of the best dribblers in France last season, Dembele took Ligue 1 by storm, scoring 12 times for a Rennes side that finished eighth. His finishing rate was also among the best in France. His inevitable summer transfer was secured by Dortmund, for whom he demonstrated his potential with a glittering solo effort against Manchester United in preseason.
Gianluigi Donnarumma, AC Milan (17)
The precocious goalkeeper is regarded by many as Gianluigi Buffon's successor, having become Milan's first choice between the sticks at 16. While the Rossoneri did concede 43 goals as they came seventh in Serie A last season, it might have been worse had it not been for Donnarumma, whose 74.3 save percentage was inferior only to four other first-choice goalkeepers in Serie A.
Breel Embolo, Schalke (19)
The Cameroon-born forward, who now represents Switzerland, scored 21 goals in 60 league fixtures for Basel before joining Schalke after the Swiss' exit from Euro 2016. Quick, strong and skillful, Embolo's ability to hold off defenders and take initiatives down the flanks makes him a suitable lone striker. The reported €20million fee forked out by Schalke reflected his growing reputation.
Jose Maria Gimenez, Atletico Madrid (21)
Few players better embody the steel and mental strength for which Atletico have become renowned under Diego Simeone. Gimenez was at the heart of the joint-best defense in La Liga history last season, keeping clean sheets alongside Uruguayan compatriot Diego Godin through sacrificial play and raw dueling power.
Alen Halilovic, Hamburger SV (20)
Halilovic spent last season on loan at Sporting Gijon, then returned to Barcelona only to be sold to Hamburg with a €10m buy-back option in 2018. A left-footed playmaker often used on the right wing, the Croatia international boasts tremendous agility and close control. And while the Lionel Messi parallels may flatter his prospects, they do create an accurate portrayal of his style.
Kelechi Iheanacho, Manchester City (19)
The Nigerian striker averaged 0.84 goals per 90 minutes over 26 league appearances last season, which made for the most clinical finishing rate after Callum Wilson and Marcus Rashford (counting those with five goals or more). The arrival of Pep Guardiola could well be good news for him, as his mobility makes him a better fit as Sergio Aguero's understudy than the more static Wilfried Bony.
Joshua Kimmich, Bayern Munich (21)
Kimmich's rise seems inexorable. Since debuting for Bayern a year ago, the defender has risen to every challenge, making 15 league starts to earn a place in Germany's Euro 2016 squad. In France, he claimed the right-back position. Like most of Pep Guardiola favorites, Kimmich is clever and versatile, and will surely continue his promising development under Carlo Ancelotti.
Anthony Martial, Manchester United (20)
The French forward plays with such maturity and composure that it's easy to forget he's still 20. His debut season at Old Trafford yielded 11 goals in 31 league games, mostly from out wide, which is not bad in a Louis van Gaal team. Martial now looks set for a regular left-wing spot under Jose Mourinho, which means a sparkling season could be in store.
Emre Mor, Borussia Dortmund (19)
Mor showcased his trickery at the Euros, where he represented Turkey despite having made youth-level appearances for Denmark, the country where he was born. Now signed by Dortmund, the attacking midfielder's arrival adds another name to a collection of sparkling dribblers that will surely make Dortmund one of Europe's most exciting teams this season.
Ruben Neves, Porto (19)
The central midfielder became the youngest captain in Champions League history last year at 18, and has become a first-team regular at the Dragao. He also became Porto's youngest goal scorer in the top flight back in 2014 at 17 years and 155 days. A deep-lying playmaker, he dictates the tempo with supreme vision and passing, and is also able to skip past players through the centre.
Saul Niguez, Atletico Madrid (21)
It looked a blow to Atletico when Arda Turan left for Barcelona last summer, but Diego Simeone responded by nurturing one of the finest talents in Spain. Saul has shined as a creative and hardworking right midfielder in El Cholo's 4-4-2 system, while chipping in with four league goals, as well as his moment of magic in the Champions League semifinal against Bayern.
Divock Origi, Liverpool (21)
The Belgium international looks increasingly suited to Jurgen Klopp's high-octane style and could become important this season given Daniel Sturridge's fitness issues. Building on a league campaign in which he started just seven times, the mobile striker scored twice from three efforts on Liverpool's recent stateside tour.
Christian Pulisic, Borussia Dortmund (17)
Pulisic is already being regarded as a candidate to become the best US player of all time. It's still early, but the prodigy from Pennsylvania has broken a series of records already, such as when he became the youngest foreign goal scorer in the history of the Bundesliga last year. A slick and intelligent winger, he seems to have the world at his feet.
Marcus Rashford, Manchester United (18)
The teenage sensation was one of the better legacies Louis van Gaal left at Old Trafford. Nearly a third of his finishes hit the net in the league last season, before lively displays at the Euros enhanced his reputation. Though Zlatan Ibrahimovic will surely start the big matches, Rashford's pace and trickery could be vital late in games as defenders start to tire.
Renato Sanches, Bayern Munich (18)
The dynamic midfielder attracted attention when joining Bayern for €35 million in May. Then the Euros showed what all the fuss was about. The former Benfica youngster grew in importance for Portugal en route to the final, and was later named the UEFA Young Player of the Tournament. With skill in abundance and tremendous energy, he seems a coup even at the significant fee he commanded.
Leroy Sane, Manchester City (20)
Only the future knows whether Sane will cut it at City, but when Pep Guardiola is among your admirers, the omens always look good. The German winger cost the Citizens a reported £37 million and earned compliments from Guardiola while at Schalke. That might have been for some of his 91 successful dribbles -- the most in the Bundesliga last season -- or his eight league goals.
Raheem Sterling, Manchester City (21)
After a dreadful summer at the Euros, Sterling faces a fight to recover flow and confidence. It won't help that City have added Nolito and Leroy Sane to a squad already featuring David Silva, Samir Nasri and Kevin De Bruyne, though Pep Guardiola has been supportive of the nimble winger. While few doubt his potential, Sterling will want to get back on track quickly.
Julian Weigl, Borussia Dortmund (20)
The defensive midfielder enjoyed an excellent breakthrough season under Thomas Tuchel. There were no goal involvements to speak of in the Bundesliga, but Weigl made the third-most tackles (40) and the second-most interceptions (63) at Dortmund to provide a stabilizing presence behind more adventurous teammates. Other notable traits included his intelligent distribution and reading of the game.
Thore Haugstad covers football from an analytics perspective for FourFourTwo and other publications/websites. He can be found on Twitter @Haugstad1006.