Tammy Abraham - Chelsea to Roma - €40m
The biggest deal done in Serie A this summer, and potentially one of the most influential.
It is very, very early days but Tammy Abraham, with his pace, selflessness and awareness, looks to have added a whole new dimension to a Roma side that finished seventh last season.
It is common knowledge that the 23-year-old Londoner required a lot of convincing to move to the Stadio Olimpico, but there is clearly the potential for Abraham to take his game to a whole under level under the tutelage of Jose Mourinho.
Admittedly, €40 million (£34m/$47m) is an awful lot of money for an unproven striker, particularly in a league financially decimated by the pandemic, but Abraham has already racked up one goal and two assists in his first two appearances for the Giallorossi.
Talk of a Scudetto challenge feels premature, but the England centre-forward might just have it in him to sustain a top-four tilt, which would represent an excellent return on Roma's investment.
David Alaba - Bayern Munich to Real Madrid - Free
Bayern Munich refused to bow David Alaba's exorbitant wage demands, but Real Madrid were only too happy to offer the Austria international a reported net salary of €12m (£10.3m/$14.2m) per year.
Los Blancos have, of course, been attempting to cut costs over the past two windows because of the economic effects of the Covid-19 crisis, but it is easy to understand why they were willing to push the boat out for Alaba.
Firstly, they had a massive hole to fill in defence following the departure of former captain Sergio Ramos and Alaba is an outrageously talented footballer capable of playing in a variety of positions.
Secondly, he was available on a free transfer and, given he is only just turned 29, Madrid understandably believe that this world-class performer still has a huge amount left to offer at the very highest level.
Hakan Calhanoglu - AC Milan to Inter - Free
AC Milan have been extremely active this summer, wrapping up deals for 11 players in total, but there is no disguising the fact that they were hit hard by the loss of two key players on free transfers.
Gigi Donnarumma's departure hit hard, because he was a youth-academy product who had become arguably the best player in the world in his position. However, Milan have, at least, replaced him well, by bringing in Mike Maignan from Lille.
There is also the fact that the Italy goalkeeper has left for Paris Saint-Germain rather than another Serie A club. Hakan Calhanoglu, though, has joined Milan's city rivals Inter, which makes his exit particularly painful for Milan.
Again, Brahim Diaz could prove an excellent substitute for the Turkey international, but the Real Madrid loanee is not yet at the same level. Few players proved as crucial to the Rossoneri's revival under Stefano Pioli as Hakan, who excelled after being deployed as a trequartista.
Worse still, he is already looking like an ideal fit for Simone Inzaghi's Inter, with Hakan having racked up a goal and an assist on his Nerazzurri debut a fortnight ago.
Memphis Depay - Lyon to Barcelona - Free
It has been a traumatic transfer window for Barcelona fans, as the full extent of the club's economic crisis became painfully clear.
As well as losing the legendary Lionel Messi to PSG for nothing, the Blaugrana were forced to shop in the bargain basement in a desperate bid to strengthen their squad.
It remains to be seen if Eric Garcia will develop into a world-class centre-half, or if Sergio Aguero will ever be injury-free, but Memphis Depay is already looking like an inspired signing.
The 27-year-old has always had the talent to shine at the highest level, of course, and Ronald Koeman knows full well how to get the best out of the versatile forward, with the pair having previously worked together in the Netherlands national team.
So, with two goals in his first three Liga appearances, Memphis might just be capable of making the coming campaign a little more bearable for Barca's beleaguered supporters.
Rodrigo De Paul - Udinese to Atletico Madrid - €35m
Rodrigo De Paul has long been regarded as one of the most talented midfielders in Serie A, and it was always expected that he would leave Udinese for one of Europe's elite clubs this summer.
What was surprising is that Atletico Madrid had such little competition for the Argentine's signature, and that they only had to pay €35m (£30m/$42m) for the privilege.
Granted, that may be considered a lot of money in the current climate, but it should prove an absolute steal, with De Paul, who was absolutely outstanding in Argentina's Copa America success, already looking right at home in Diego Simeone's midfield.
Matteo Guendouzi - Arsenal to Marseille - Loan
Upon announcing the arrival of Matteo Guendouzi on a season-long loan with a view to a permanent transfer, Marseille described the midfielder as "a player of character".
Arsenal boss Mikel Arteta might well disagree, given he had a major problem with the 22-year-old's attitude.
It was always clear, though, that Guendouzi had serious talent and he is already showing it again at the Velodrome.
Indeed, the former Lorient ace has been rewarded for an impressive start to his Marseille career with a call-up to the France squad, meaning after a tough period in his career, he is now in line to make his first appearance at senior level for his country.
Achraf Hakimi - Inter to PSG - €60m
PSG moved quickly and cleverly to take advantage of the financial crisis at Inter this summer by signing Achraf Hakimi for an initial €60m (£52m/$71m) and a potential €10m (£9m$12m) in add-ons.
Now, the Ligue 1 outfit may have made more high-profile deals during a remarkable window, but make no mistake about it: Hakimi is a potential superstar.
An outstanding attacking full-back, the Moroccan was a key cog in Antonio Conte's winning machine at Inter, who romped to a first Serie A title in 11 years, and, at 22, he is only going to get better.
Manuel Locatelli - Sassuolo to Juventus - Loan
Juventus' negotiations with Sassuolo over the transfer of Manuel Locatelli to Turin may have been laborious but, at the end of the day, they could not have worked out much better for the Bianconeri.
Midfield has been a problem area for the Old Lady for years now, so the arrival of one of the stars of Italy's Euro 2020 triumph is an undoubted cause for celebration.
Juve have also structured the deal superbly, as they are get Locatelli on loan for two years before being obliged to pay €25m (£21.5m/$29.5m) and a potential €12.5m (£11.75m/$14.75m) in add-ons in the summer of 2023, by which point the Italians expect to be on a much firmer financial footing.
The 23-year-old may not be the classic regista that many pundits believe that Massimiliano Allegri needs, but Locatelli will offer a mix of dynamism and composure that the Bianconeri sorely lacked last season.
Lionel Messi - Barcelona to PSG - Free
The move that turned the entire transfer window on its head.
Lionel Messi was all set to sign a new deal with Barcelona when it emerged that the cash-strapped Catalan club were not in a position to register his contract because they were still in breach of La Liga's salary regulations.
After a tearful goodbye, the Argentine swiftly signed for Paris Saint-Germain, who are now in possession of one of the most fearsome-looking forward lines the game has ever seen.
Messi may be 34 but he remains a phenomenon, as he underlined once again by claiming both the Golden Ball and the Golden Boot after Argentina's Copa America triumph in July.
Losing arguably the greatest player in history for nothing is a hammer blow for Barcelona, but an incredible coup for PSG, who will now be supremely confident of winning a first Champions League given they have also picked up Sergio Ramos, Gini Wijnaldum and Gigi Donnarumma on frees.
Ilaix Moriba - Barcelona to RB Leipzig - €16m
Barcelona may well feel that they had no other option but to sell Ilaix Moriba, who was refusing to extend his contract with the club.
In that sense, maybe a €16m (£13m/$19m) fee, which could rise to €22m (£19m/$26m) with bonuses, is not a bad deal for the Blaugrana, who are also entitled to a 10 percent share of any future transfer.
But there is no denying that they would have dearly loved to have held on to one of the most exciting prospects to have come out of La Masia in years, and that the teenager could well develop into a top midfielder at RB Leipzig, a club renowned for nurturing young talent.
Saul Niguez - Atletico Madrid to Chelsea - Loan
If Romelu Lukaku fires Chelsea to the Premier League title, then his massive £98m ($136m) transfer fee will be regarded as money well spent by the Blues.
Thomas Tuchel's side were clearly lacking in the forward department and, after two sensational seasons under Antonio Conte at Inter, the brilliant Belgian looks belatedly ready to become an icon at Stamford Bridge.
However, Chelsea's loan deal for Saul Niguez could be an even better bit of business. The Spain international has, by his own admission, not been himself for the past two years, but he feels that's because he was no longer being played in central midfield by Atletico Madrid coach Diego Simeone.
Chelsea may have a fairly settled side but they have promised Saul game time in his preferred position, and if he gets back to anything like his best form, the Londoners will not even think twice about activating his £30 million ($41m) purchase clause at the end of the season.
Cristiano Ronaldo - Juventus to Man Utd - £20m
Manchester United would arguably have been better served by the acquisition of a central defensive midfielder this summer, but the return of Cristiano Ronaldo to Old Trafford has unsurprisingly sent the club's supporters into a frenzy.
The Portuguese superstar may be 36 now and needs an attack to be constructed around him, given he contributes next-to-nothing from a defensive perspective.
But Ronaldo guarantees goals, making his £20 million ($28m) fee look like a bargain.
We are talking about arguably the most well-rounded goalscoring machine in the history of football, and he arrives in Manchester having won last season's Capocannoniere award in Serie A, and claimed the Golden Boot at Euro 2020.
Ronaldo's second stint at United may not be quite as successful as his first, but it would still be a massive surprise if he does not make a significant impact on Ole Gunnar Solskjaer's squad.
Andre Silva - Eintracht Frankfurt to RB Leipzig - €23m
Andre Silva's time at AC Milan may have been a disaster, but the forward has rebuilt his reputation in the Bundesliga, which is why RB Leipzig were so happy to have persuaded Eintracht Frankfurt to part company with the Portuguese this summer.
"He is a good fit for our team and our style of football both as a person and as a player," Leipzig technical director Christopher Vivell stated after Silva's €23m move to the Red Bull Arena was confirmed.
"He is a complete forward: he can finish with both feet, he's strong in the air and he's technically gifted."
Silva's strike-rate is certainly impressive, with the former Porto prodigy having hit 40 goals in just 57 Bundesliga appearances for Eintracht.
He has already opened his account for Leipzig, too, so the early signs are that the 25-year-old will continue terrorising defences in Germany's top flight for the foreseeable future.
Dayot Upamecano - RB Leipzig to Bayern Munich - €42.5m
It was clear that the Bayern Munich defence would need an overhaul this summer, with both David Alaba and Jerome Boateng heading for the exit door.
The Bavarians, then, moved quickly to snap up one of the most highly rated centre-backs in Europe, announcing all the way back in February that they had agreed to meet the €42.5m (£37m/$52m) buy-out clause in Dayot Upamecano's RB Leipzig contract.
Upamecano is by no means the finished article - he remains prone to passing and positional errors - but the potential is there for all to see and, at 22, he could easily establish himself as a mainstay in the Bayern starting line-up for the best part of a decade.
Raphael Varane - Real Madrid to Man Utd - £40m
It says much about the nature of the English transfer market that Arsenal had to pay £50m ($69m) for Ben White, who has been capped twice by England, whereas Manchester United were able to sign Real Madrid's World Cup-winner Raphael Varane for £10m ($13.8m) less.
Varane made an impressive debut at Wolves on Sunday, dominating in defence as well as teeing up the match-winner for Mason Greenwood, and that should not be in the least bit surprising of course.
The Frenchman may have endured a dip in form after Russia 2018, but the four-time Champions League winner remains one of the best centre-halves in the business and, with his pace and intelligent positioning, he should prove an excellent partner for Harry Maguire.