Chelsea have completed the signing of France international midfielder Tiemoue Bakayoko from Monaco on a five-year contract for a reported fee of £40 million.
Bakayoko, 22, becomes Chelsea's third signing of the summer, following Willy Caballero and Antonio Rudiger to Stamford Bridge as head coach Antonio Conte looks to bolster his squad in preparation for a return to the Champions League.
The Blues have been on the verge of completing the deal for several weeks, but a knee injury sustained by Bakayoko in the final weeks of last season delayed his medical and could reportedly mean that he is not fit in time for the first weeks of the new campaign.
Bakayoko told the club's official website: "I am very happy to be here and to join this great team. I have grown up watching Chelsea. Signing was the natural thing for me because it is a club I loved very much in my childhood.
"I am now looking forward to evolving under this fantastic coach and alongside so many great teammates."
Bakayoko joined Monaco from Rennes in the summer of 2014 and established himself as a key figure under coach Leonardo Jardim last season, making 46 appearances in all competitions to help his team claim the Ligue 1 title ahead and reach the Champions League semifinals.
At £40m, he is the most expensive midfield signing in Chelsea's history and the second-most lucrative overall, behind only the £50m paid to acquire Fernando Torres from Liverpool in January 2011.
"Tiemoue was outstanding as Monaco emerged as one of Europe's best sides last season and we are delighted to welcome him to Chelsea," technical director Michael Emenalo said.
"Despite his young age he already has plenty of experience as a French title winner and having competed in the latter stages of the Champions League, we believe he will add considerable quality to our squad."
Central midfield is only one area of Chelsea's squad that Conte has identified as in need of reinforcement this summer. The Blues are also in talks over the signing of wing-back Alex Sandro from Juventus, reportedly for a club-record £61m, and remain in pursuit of a striker to replace Diego Costa.
Alvaro Morata and Andrea Belotti both feature on Conte's wish list, and the Blues are expected to turn their attentions to the Real Madrid striker after losing out to Manchester United in the battle to sign top target Romelu Lukaku from Everton.