Mikhail Khodorkovsky was first arrested in October 2003 on charges of tax evasion, fraud and embezzlement.
He was previously the head of oil giant Yukos and was one of Russia's richest men.
Khodorkovsky was jailed in 2005 for eight years and was due to be released in 2011. But in 2009 he was put on trial again, along with his jailed partner Platon Lebedev, on further charges of embezzlement and money laundering - and both were declared guilty again.
Khodorkovsky made his fortune - estimated at more than $15bn - from the controversial post-Soviet privatisation of state assets.
Before his arrest, he was not just rich but very influential. His lawyers have always maintained that the charges against him are politically motivated, carried out on the orders of senior figures in the Kremlin who objected when his activities went into the political arena.
Khodorkovsky had provided funding to opposition parties and acquired the rights to publish the prestigious Moskovskiye Novosti newspaper.
While still in charge of Yukos, he maintained that he was not tied to any particular party.
However he was vocal of his support for President Vladimir Putin's political opponents.
When asked about Khodorkovsky on live television before the second trial verdict was read out, Putin compared Khodorkovsky to convicted US fraudster Bernard Madoff, saying: "a thief must be in jail".
A resident of Moscow and the son of two engineers, Khodorkovsky studied at Mendeleyev Chemistry Institute and began his career as a Soviet-era Communist Party member, running a computer import business under the wing of Komsomol, the party's youth movement in the 1980s.
In 1987 - four years before the fall of the Soviet Union - he founded what would become Menatep, one of post-Soviet Russia's first private banks.
It's not me and Platon Lebedev who are now standing trial, it's all the Russian people.
He made his first millions in the 1990s when the bank acquired massive amounts of shares in companies that were privatised for low prices.
In 1995, Khodorkovsky acquired Yukos at a state auction at a price of $350m. Yukos had come close to folding but became Russia's second biggest oil company, pumping one in every five barrels the country produced.
It began publishing its accounts to international standards and was soon seen as one of Russia's most transparent, well-run companies. Khodorkovsky even served as deputy fuel and oil minister during Boris Yeltsin's presidency.
But after Putin came to power, Khodorkovsky's increasing political activities aroused suspicions of his own ambitions.
In 2003, fellow Yukos shareholder Platon Lebedev was arrested for fraud in a step widely seen as a warning to Khodorkovsky to keep out of politics.
Four months later, armed security forces detained him on his private jet at an airport in Siberia.
Tax police filed enormous claims for unpaid taxes against Yukos and, unable to pay, the company filed for bankruptcy in 2006, and its production assets were sold off at state-run auctions.
Khodorkovsky and his fellow inmate Lebedev served much of their sentences at a Soviet-era labour camp in the Chita region of eastern Siberia, 4,700km east of Moscow.
They were moved to Moscow in early 2009 to face their second trial, which involved charges of misappropriating nearly $27.7bn and laundering almost $12.5bn.
As the trial itself drew to an end, Khodorkovsky himself predicted that his release was unlikely but warned that the fate of all Russians rested on the case.
"It's not me and Platon Lebedev who are now standing trial, it's all the Russian people," he told the court in his final address.
He went on to say that he had no wish to die in jail but "if that is what is needed, I have no hesitation".
In a surprise move, Putin decided to pardon Khodorkovsky in December 2013. After leaving prison, he flew to Berlin where his ailing mother has been undergoing medical treatment.