The summer and winter transfer windows were introduced by UEFA in 2002.
Brought in to "ensure a partial stabilisation of club squads during the season" and to stop the confusion which followed the Bosman ruling, the windows meant European clubs had to plan their transfer business more effectively.
Most big transfers still happen in the summer, with a player more likely to move or be allowed to leave at the conclusion of a season.
But over the years, there have been some notable deals completed in January. Here, a look at the biggest transfers in the winter window...
Alex Teixeira (Shakhtar Donetsk to Jiangsu Suning)
A key player in a dominant period for Shakhtar Donetsk in Ukrainian football, the Brazilian helped the Miners to five league titles and three cups in five-and-a-half seasons.
The forward was eventually sold to Chinese side Jiangsu Suning in January 2016 for €50 million. Teixeira won one Chinese Super League crown in five seasons at the club, before joining Beşiktaş in 2021 and then returning to Brazil the following year.
Luis Díaz (Porto to Liverpool)
After impressing in two-and-a-half seasons in Portugal, Luis Díaz left Porto for Liverpool in a €52 million transfer in January 2022.
The Colombian winger quickly became a popular player for the Reds, helping Jürgen Klopp's side to several domestic honours and starting in the 2022 Champions League final loss to Real Madrid in Paris.
Ferran Torres (Manchester City to Barcelona)
Ferran Torres left Manchester City for Barcelona for a fee of €55 million in January after falling out of favour in his second season, mainly due to injury problems, under Pep Guardiola.
A LaLiga and Supercopa winner with Barça in 2022/23, Torres was also part of Spain's squad at Euro 2024, though he did not feature as La Roja beat England in the final in Berlin.
Fernando Torres (Liverpool to Chelsea)
Fernando Torres formed an almost telepathic understanding with Steven Gerrard at Liverpool and was adored by the Reds fans.
But something was broken by January 2011. Struggling for form after he pushed his body to the limit to play the 2010 World Cup for Spain, the striker was unhappy. He joined Chelsea in a €58.5 million deal in the winter window and went on to win the Champions League, a Europa League and an FA Cup with the Blues, but was never quite the same player and returned to boyhood club Atlético Madrid in 2015 after a short spell at AC Milan.
Oscar (Chelsea to Shanghai SIPG)
A Premier League winner with Chelsea, Oscar also helped the Blues to a League Cup and a Europa League in four-and-a-half seasons at Stamford Bridge.
One of a number of stars to move to the Chinese Super League amid a big investment from that competition, the Brazilian midfielder joined Shanghai SIPG in a €60 million transfer in January 2017 and led the club to a series of titles over the next few years.
Diego Costa (Chelsea to Atlético Madrid)
Diego Costa left Atlético Madrid to join Chelsea in a €35 million deal in the summer of 2014 and the Brazil-born forward's goals helped the Blues win two Premier League titles in three seasons.
Informed in a text message that he was not in the plans of manager Antonio Conte for the 2017/18 season, Costa eventually moved back to Atleti in a €60m transfer in January 2018. However, he was much less successful second time around and returned to Brazil in 2021.
Pierre-Emerick Aubameyang (Borussia Dortmund to Arsenal)
After four-and-a-half seasons and over 100 goals for Borussia Dortmund, Pierre-Emerick Aubameyang was sold to Arsenal for a fee of €63.8 million in January 2018.
Initially a hit with the Gunners, Aubameyang fell out of favour with manager Mikel Arteta and was sold to Barcelona in January 2022, before returning to the Premier League with Chelsea in the summer.
Christian Pulisic (Borussia Dortmund to Chelsea)
Christian Pulisic left Borussia Dortmund to join Chelsea in January 2019 in a €64 million transfer.
The deal made Pulisic the most expensive North American player of all time, but the winger was sold to AC Milan for just over a third of that amount in the summer of 2022.
Bruno Fernandes (Sporting CP to Manchester United)
Manchester United paid Sporting CP a fee of around €65 milion to take Bruno Fernandes to Old Trafford in January 2020, with the total amount possibly rising to as much as €80m.
Virtually ever-present under several managers since then, Fernandes helped United win the League Cup in 2023 and the FA Cup the following season.
Aymeric Laporte (Athletic Club to Manchester City)
Manchester City paid Aymeric Laporte's release clause of €65 million to take the centre-back to the Etihad from Athletic Club in January 2018.
Laporte quickly became a key player for City, helping the Sky Blues to five Premier League titles and a treble triumph in 2022/23, before moving to Saudi Arabia with Al-Nassr.
Gonçalo Ramos (Benfica to Paris Saint-Germain)
Benfica have made huge profits by selling players to top clubs across Europe in recent years and Gonçalo Ramos is one of the Eagles' biggest recent sales.
The striker left Lisbon for Paris Saint-Germain initially on loan, before completing a €65 million permanent deal in January 2024.
Mykhailo Mudryk (Shakhtar Donetsk to Chelsea)
Both Arsenal and Chelsea were keen to sign Mykhailo Mudryk from Shakhtar Donetsk in January 2023.
The Gunners were ultimately put off by the huge asking price in a bidding war, but the Blues were not, paying €70 million for the 22-year-old winger.
Dušan Vlahović (Fiorentina to Juventus)
Amid interest from top clubs all over Europe, Juventus moved quickly to sign one of the hottest properties in Serie A in January 2022.
Dušan Vlahović moved to Juve from Fiorentina on his 22nd birthday for a total fee of around €81.6 million and the Serbian scored the winner against Atalanta to clinch his first trophy, the Coppa Italia, in May 2024.
Virgil van Dijk (Southampton to Liverpool)
With Philippe Coutinho sold to Barcelona for a huge fee in January 2018, Liverpool moved to strengthen their back line by bringing in Virgil van Dijk from Southampton.
The Reds paid €84.7 million to the Saints for the Dutch defender. It was a huge fee at the time, but his vast contribution to their successes in recent years make it one of the club's best-ever deals.
Enzo Fernández (Benfica to Chelsea)
Enzo Fernánez caught the eye in Argentina's run to World Cup victory at Qatar 2022 and the midfielder was on the move the following month.
Having spent only six months at Benfica, the Portuguese club demanded his release clause be paid in order for a transfer to go ahead and Chelsea duly obliged, shelling out €121 million for the 22-year-old. The move was a British record transfer fee at the time.
Philippe Coutinho (Liverpool to Barcelona)
After Paris Saint-Germain triggered Neymar's €222 million release clause to take Neymar to the Parc des Princes in the summer of 2017, Barcelona had money to spend.
But the Catalan club did not spend it wisely. After a huge outlay to sign Ousmane Dembelé late in the summer, Barça splashed €135 million on Philippe Coutinho. The transfer ultimately strengthened Liverpool more than Barça, with the Reds using the funds to address key weaknesses in their squad and winning the Champions League the following season. Coutinho, meanwhile, was sent out on loan to Bayern Munich and eventually sold to Aston Villa at a huge loss.
