The Ghanaian re-joins former Blues boss Jose Mourinho, who brought him to England in 2005, at the Bernabeu.
The 29-year-old has often been touted as a box-to-box midfielder for his ability to exert boundless energy in supporting offensive and defensive play.
Essien can also play as a centre-back.
Essien began his career playing for Liberty Professionals in his home country. In 2000, he moved to France joining SC Bastia. Essien spent three seasons at the club appearing in over 60 matches before joining league champions Olympique Lyonnais in 2003.
At Lyon, Essien won back-to-back league titles in 2003–04 and 2004–05 and also played in the UEFA Champions League for the first time. During his five-year stint in France, he acquired French citizenship.
In 2005, Essien signed with Chelsea for a fee of £24.4 million and, at the time of his signing, was the most expensive African footballer in history.
At Chelsea, Essien helped the club win the Premier League in 2006 and 2010, as well as three FA Cups and one Football League Cup. In 2008, he appeared in the UEFA Champions League Final. He has won the Chelsea Goal of the season award twice, in the 2006–07 and 2008–09 seasons.
A Ghanaian international, Essien started at youth level as he represented his country at the 1999 FIFA U-17 World Championship and 2001 FIFA World Youth Championship with the latter team finishing as runner-up.
Essien made his senior team debut in January 2002 and has represented his nation at three Africa Cup of Nations tournaments and the 2006 FIFA World Cup, where Ghana reached the Round of 16. He is often referred to as the Bison for his tough tackling style, boundless energy and physical presence on the pitch.