Here are the biggest summer transfer needs for big-name clubs in the Bundesliga, English Premier League, La Liga, Ligue 1 and Serie A.
The clubs selected for this slideshow are AC Milan, Arsenal, Atletico Madrid, Barcelona, Bayer Leverkusen, Bayern Munich, Borussia Dortmund, Chelsea, Inter Milan, Juventus, Lille, Liverpool, Manchester City, Manchester United, Monaco, Paris Saint-Germain and Real Madrid.
Lille Transfer Need: Centre-Forward
Lille's defence at times has been impenetrable this season, hence why the club have only conceded 16 goals in 25 league games, but 10 Ligue 1 teams, including 14th-placed Montpellier, have scored more goals than Lille.
While Salomon Kalou has had some highlight-reel moments, seven goals in 25 games is not a good return.
Monaco Transfer Need: Right-Back
Andrea Raggi, who is better at centre-back, is a serviceable right-back, but with Monaco's resources, they need to have world-class players in every position.
In an ideal world, Monaco sign Rio Ave loanee Fabinho permanently in the summer transfer window, since he has the potential to transition into an elite attacking right-back.
Should that not work, Monaco need to target a right-back.
Paris Saint-Germain Transfer Need: Left Forward
In 47 Ligue 1 games, Ezequiel Lavezzi has only scored seven times and created four goals. He is not performing at the level expected of him.
The greatness of Zlatan Ibrahimovic, who scored 19 goals in Ligue 1 and 10 goals in the UEFA Champions League, has ensured Lavezzi's lack of productivity has not become a serious talking point.
AC Milan Transfer Need: Centre-Back
Milan are ninth in Serie A, so the club needs an overhaul and centre-back should be the primary summer transfer need.
This season, Milan have tried six centre-backs (Adil Rami, Cristian Zaccardo, Cristian Zapata, Daniele Bonera, Matias Silvestre and Philippe Mexes), yet 10 Serie A teams including 15th-placed Cagliari have conceded less goals than Milan (35 goals conceded in 24 games).
Inter Milan Transfer Need: Centre-Back
Andrea Ranocchia's tackles-per-game ratio has decreased by 1.8 compared to last season, and he has been conceding daft free-kicks.
In his last five Serie A games, he has received four yellow cards and his display in the 4-2 defeat to Napoli was concerning because he was out of position constantly.
This is why Inter are targeting Nemanja Vidic, per Simone Bargellini and Nadia Carminati at Sky Sports.
Juventus Transfer Need: Left-Back
Juventus' primary formation is a 3-5-2, but the club started with a back four in the UEFA Champions League group games against Real Madrid.
On those two occasions, the two left-backs were Angelo Ogbonna and Kwadwo Asamoah.
Both are short-term options at left-back, but Ogbonna is a centre-back and Asamoah is a utility player.
Should Juve manager Antonio Conte want to use a 4-3-3 more predominantly next season, he needs to sign a full-time left-back, preferably an attacking one as he would make an easier transition into a wing-back in a 3-5-2.
Atletico Madrid Transfer Need: Goalkeeper
Thibaut Courtois will not have his loan at Atleti extended into a permanent transfer and will return to Chelsea next season, per ESPN.
Without Courtois, Atleti will need to replace him if they do not view backup Daniel Aranzubia as No. 1 quality.
Barcelona Transfer Need: Goalkeeper
Victor Valdes wants to challenge himself and break out of the Catalan bubble.
Last September, Barca manager Gerardo Martino confirmed the club would respect Valdes' wishes, per Football Espana: "There's no chance of him [Valdes] renewing his contract."
Borussia Monchengladbach keeper Marc-Andre ter Stegen is the favourite to succeed Valdes, per Tom Conn at Inside Spanish Football.
Real Madrid Transfer Need: Left-Back
Out of 24 La Liga games this season, Fabio Coentrao has only played five league games, which is around 20.8 percent—he is not reliable enough.
With Coentrao's body consistently breaking down, Real cannot afford to keep him when he is spending segments of the campaign in the medical room.
Both Alvaro Arbeloa and Nacho Fernandez have filled in, but are not full-time left-backs, so Real are exposed when first-choice left-back Marcelo is out with injury.
Arsenal Transfer Need: Centre-Forward
Olivier Giroud's offensive output is good: 10 goals and six assists in 24 Premier League games equates to 16 total goals.
His problem is finishing since 7.8 shots per goal is inefficient.
He needs to take charge in the big games as he did not score in a 2-0 defeat to Chelsea, a 6-3 defeat to Manchester City and a 5-1 defeat to Liverpool.
Therefore signing a high-quality No. 9 will either push Giroud to up his game or be an upgrade over the Frenchman.
Chelsea Transfer Need: Left-Back
Ashley Cole has been warming the benches this season, having lost his starting position to Cesar Azpilicueta, who was signed as a right-back.
With Ryan Bertrand loaned out to Aston Villa and Cole's contract expiring at the end of the season, the Blues should target a left-back in the summer transfer window.
Liverpool Transfer Need: Left-Back
Aly Cissokho, on loan from Valencia, is philosophical about his future at the club.
"Critics? I am used to not winning unanimous support," Cissokho said, per David Anderson at the Mirror. "If I have to leave Liverpool, I will leave."
Jose Enrique is recuperating from knee surgery and makeshift left-back Jon Flanagan moved back to right-back, hence why Liverpool should be scouting left-backs right now.
Manchester City Transfer Need: Centre-Back
Martin Demichelis had a Renaissance in La Liga, but he has been caught out in the Premier League.
He is flat-footed, he is reactionary, he is slow and is easily bypassed.
City need to have world-class centre-backs, and Demichelis should not be a week in, week out starter.
This is why City attempted to sign Porto's Eliaquim Mangala on the January transfer deadline day, per Mark Ogden at the Telegraph.
Manchester United Transfer Need: Central Midfielder
Tom Cleverley believes the vitriolic criticisms of his performances this season are unfair.
"My job goes under the radar at times," Cleverley said, per Oliver Holt at the Mirror. "I am not a player who's going to beat three or four people and stick it in the top corner or go round tackling people like Roy Keane."
Therein lies the problem with Cleverley: He is not a dominant or commanding central midfielder such as Roma's Kevin Strootman, who has made 76 tackles, scored five goals and registered six assists.
United must upgrade from Cleverley.
Bayern Munich Transfer Need: N/A
Bayern Munich are not only unbeaten in the Bundesliga this season, but Pep Guardiola's team have scored the most goals (57) and conceded the least amount of goals (nine).
Bayern do not have a transfer need as they have the best squad in Europe.
Bayer Leverkusen Transfer Need: Right Forward
Sidney Sam has scored seven times and created seven goals in 15 league games this season, but he has agreed to join Schalke.
Robbie Kruse, the backup right forward, suffered an anterior cruciate ligament tear, so his future is uncertain.
While the versatile Gonzalo Castro can play as a right forward, his influence in midfield is missed.
Borussia Dortmund Transfer Need: Centre-Forward
Robert Lewandowski, Dortmund's leading goalscorer this season, will be joining Bayern next season.
Dortmund need to find a No. 9 who can emulate his 24 goals last season and 14 goals thus far this season, which will be a tough task.
Dortmund can either do it in-house, moving right attacking midfielder Pierre-Emerick Aubameyang into the centre-forward slot, or they can make a notable summer transfer window acquisition.
Former FIFA World Cup winner Lothar Matthaus believes Dortmund will sign Hertha Berlin's Adrian Ramos, per Stefan Coerts at Goal.com.
Ramos is tied with Lewandowski on 14 goals for most Bundesliga goals this season.