The transfer window is the period during the year in which a football club can transfer players from other countries into their playing staff. Usually, the January transfer window opens on the 1st of January and closes on the 31st of January.
This is a very crucial time for all the clubs to reassess their squads and bring in reinforcements to shore up all the weak areas.
However, teams are usually reluctant to spend big money on players during this time due to a variety of factors. Firstly, the players coming in get very little time to acclimatize themselves with the players and the style at their new club.
Also, many of the players are cup-tied - for example, a player who has played in the Champions League or the Europa League in the present season cannot play for any other club in the same European competition in the same season.
Still, there are exceptions and some of the biggest transfers in football history have taken place in the January transfer window. So, who are the most expensive signings in January over the years?
Here is a look at the top 5;
#5 Lucas Moura - Sao Paulo to Paris Saint-Germain (£38 million)
Moura joined the French champions in a big money move from Sao Paulo in the year 2013. He had established himself as a talented youngster having broken into the Brazilian first team. His time at Sao Paulo saw him play 86 matches which included 22 goals and 19 assists.
People might have expected a young player with the burden of a huge transfer fee to initially to struggle to adapt to new conditions but Lucas proved his detractors wrong and soon established himself as an integral part of PSG's first eleven.
However, with the arrival of Neymar and Kylian Mbappe, Lucas has found games hard to come by this season and might soon be on his way out of the French capital.
He has performed reasonably well during his time at Paris scoring 46 goals and providing 50 assists in the 229 matches he has played so far.