The coronavirus pandemic affected every club's finances last summer, but clubs in Europe's top five leagues still spent over €4.6 billion in the transfer window.
However, the popularity of free agents has seen a meteoric rise the last few years, and more importantly now, as clubs continue to struggle financially. Some of Europe's greatest players are also some of the most valuable free agents in history, with three of the top five transfers taking place this summer.
Here are the top 10 market values of the most valuable free agents in history.
10. Hakan Calhanoglu - Milan to Inter - €35m (2021/22)
Calhanoglu crossed the Milan city divide to move to Inter on a Bosman earlier this summer. Last season, he helped Milan to their best league finish in seven years, and admitted that Inter were "the only club" to give him the sort of attention that sways a player to make this sort of move.
9. Adrian Rabiot - PSG to Juventus - €35m (2019/20)
Rabiot left PSG and joined Juventus on a free in 2019, after it became clear he wouldn't be signing a new deal at the Parc des Princes. The Frenchman starred for France at Euro 2020, and according to reports, is set to play a huge role at Juventus next season.
8. Michael Ballack - Bayern Munich to Chelsea - €35m (2006/07)
Ballack arrived at Stamford Bridge in a three-year deal in 2006, and was reportedly the highest-paid player at the time in the Premier League, having signed a €190,700-per-week deal. The German spent four years in west London before heading back to Germany in 2010 to join his former club Bayer Leverkusen.
7. Aaron Ramsey - Arsenal to Juventus - €40 (2019/20)
Ramsey signed a four-year contract with Juventus in 2019 on a £400,000-a-week deal, making him the highest-earning British player ever based on basic salary. However, things didn't go according to plan for the Welshman in Turin, who has been linked with an exit and a return to Arsenal has been mooted this summer.
6. Leon Goretzka - Schalke to Bayern Munich - €40 (2018/19)
After spending five years at Schalke, Goretzka made the switch to Bayern, following in the footsteps of Manuel Neuer, who made a similar move in 2011. Manchester United enquired about his availability this summer, but the midfielder insisted he wants to stay at Bayern as they look to reach an agreement over a new contract.
5. Stefan de Vrij - Lazio to Inter - €40 (2018/19)
De Vrij signed a five-year contract to join Inter from Lazio after four years at the Stadio Olimpico. The defender was in fine form for the Nerazzurri last season as they lifted the Serie A title, and will get his first opportunity to work closely with new Inter head coach Simone Inzaghi, who he also worked with at Lazio for a brief period.
4. Memphis Depay - Lyon to Barcelona - €45 (2021/22)
Depay finally made his long-awaited move to Barcelona earlier this summer on a one-year deal. The Frenchman was touted to make a switch to Camp Nou last summer only for the move to fall through. Ronald Koeman is keen to work with his compatriot after having managed him while he was in charge of the Holland national team.
3. Robert Lewandowski - Borussia Dortmund to Bayern Munich - €50 (2014/15)
Lewandowski swapped Dortmund for Bayern on a five-year deal following Mario Gotze, who made the switch in 2013. Since then, the Pole has gone on to make a name for himself as one of the best strikers in the world, and Premier League champions, Manchester City, are eyeing up a move for the 32-year-old as a potential replacement for Sergio Aguero.
2. David Alaba - Bayern Munich to Real Madrid - €55 (2021/22)
Alaba made the high-profile move to Real Madrid earlier this summer after having failed to agree on a new deal with Bayern. The Austrian comes into a Los Blancos side that saw their club captain, Sergio Ramos, join Paris Saint-Germain on a free transfer last week.
1. Gianluigi Donnarumma - Milan to PSG - €60 (2021/22)
Donnarumma became the second-youngest goalkeeper ever to debut in Serie A, and has gone on to become one of the best goalkeepers in the world right now. His heroics at Euro 2020 helped Italy claim the European crown for a second time, and after failing to agree on a new deal with Milan, the 22-year-old signed for PSG last week.