From TheChels.info - The Chelsea Football Club Wiki
Roman Arkadyevich Abramovich (born 24 October 1966) is a Russian businessman and main owner of the private investment company Millhouse LLC. He has been the owner* of Chelsea Football Club since the summer of 2003.
In June 2003, Abramovich became the owner of the companies that control Chelsea Football Club. Former owner of the club was Ken Bates, who then bought Leeds United with the money gleaned from sale. The club also embarked on an ambitious programme of commercial development, with the aim of making it a worldwide brand, and announced plans to build a new state-of-the-art training complex in Cobham, Surrey.
Chelsea finished their first season after the takeover in second place in the FA Premier League, from fourth the previous year, and reached the semi-finals of the UEFA Champions League. For the 2004-05 season a new manager, José Mourinho, was recruited, and Chelsea ended the following season as league champions. Since the takeover the club have won eight major trophies - the Premier League three times, the FA Cup three times and the League Cup twice; the most successful period in the club's history.José Mourinho appeared in the press which was due to various arguments between the two men regarding the appearances of certain players, notably Andriy Shevchenko. In the early hours of 20 September 2007, José Mourinho announced his exit as Chelsea manager by mutual consent with the club following a meeting with the board.  Former Israel coach and Chelsea's director of football, Avram Grant, was named as his replacement.  Ever since Grant had joined Chelsea (in the summer of 2007) there had been rumours of friction between him and Mourinho. Mourinho reportedly told Grant not to interfere in team affairs but with Abramovich's backing, Grant's profile at the club rose after he was made a member of the board. This event apparently did not go down well with Mourinho and may have contributed to his surprise exit.  Avram Grant led Chelsea to the position of runners-up in both the English Premiership and European Champions League - second to Manchester United on both accounts. Nevertheless, on May 24, Grant was sacked as manager by Abramovich. . On 11 June 2008, it was announced that Luiz Felipe Scolari would be taking over as manager on 1 July 2008. Scolari was sacked as Chelsea manager on 9 February 2009. In February 2009, Guus Hiddink was appointed manager of the club, but returned to his permanent position as manager of the Russian national team following the conclusion of the English season being replaced by Carlo Ancelotti. Ancelotti won the double in his first season in charge, but was sacked after a relatively disappointing second season. In June 2011 André Villas-Boas became the seventh manager of the Abramovich era. Villas-Boas' reign was shortlived, but in 2012 Abramovich saw his dream of seeing Chelsea become European champions fulfilled, former player and Assistant First Team Coach Roberto Di Matteo guiding the club to victory in Munich. By the summer of 2013, Abramovich and José Mourinho had clearly put any disagreements behind them, the Portuguese returning to Stamdord Bridge as manager.
* Chelsea FC plc is the company which owns Chelsea Football Club. The ultimate parent company of Chelsea FC plc is Chelsea Limited and the ultimate controlling party of Chelsea Limited is Roman Abramovich.