By Stephen Mulvey
BBC News Online
Mikhail Khodorkovsky has been arrested because someone does not like him - but why they do not like him is not yet clear.
There are any number of reasons, providing fertile ground for conspiracy theorists - who have always flourished in Russia - and giving Kremlinologists a chance to come out of retirement.
Discerning motives in the murky Yukos affair is a tricky business
Has Mr Khodorkovsky broken the law? Maybe, but most Russian oligarchs bent the rules when they wheeled and dealed, grabbed assets, and got rich quick in the 1980s and 1990s.
So why are they picking on him?
President Vladimir Putin appeared to offer the oligarchs a deal when he became president in 2000: stay out of politics, and we will not rake over your past misdeeds.
The most widespread explanation for Mr Khodorkovsky's troubles is that he broke the terms of the deal by getting involved in politics.
This seems to have been the reason for the hounding of two other oligarchs, Vladimir Gusinsky and Boris Berezovsky, both now in exile.
Mr Gusinsky owned a national television station, NTV, which broadcast a view of the news at odds with the Kremlin's.
Mr Berezovsky also has media holdings, and was just too addicted to political meddling. They both had to go.
Other oligarchs such as Vladimir Potanin, Mikhail Friedman and Roman Abramovich remained politically clean, concentrating on rebuilding their empires after the 1998 economic collapse, and were left alone.
According to this theory, Mr Khodorkovsky's mistake was to start openly funding political parties, with parliamentary elections due in December, and presidential elections next year.
It cannot have helped that his name was floated as a possible future president of Russia in 2008.
Another theory focuses on splits in the Kremlin.
Mr Putin was thrust into the presidency by his predecessor, Boris Yeltsin, who plucked him from the ranks of the "family" - his closest circle of friends, advisers and financial backers, including Boris Berezovsky and Roman Abramovich.
Vladimir Putin seems to have stood to one side in the Yukos affair
Mr Putin thus represents the family - but he also represents an entirely different political clan, with its roots in the security services and his home town, St Petersburg.
He once worked for both the KGB and for the St Petersburg city administration, and as he rose to power, he elevated many trusted ex-colleagues to key positions. This "law and order" clan helped him to keep the oligarchs - including those with "family" connections - on a tight leash.
It is widely assumed that this "law and order" faction is leading the fight against Mr Khodorkovsky - but again, there are various theories about its motives.
One is that the clan is frustrated by its failure to acquire real economic and financial power to go with its political clout, and decided to dismantle and redistribute Mr Khodorkovsky's empire, because it is the biggest in the country.
Another is that it wants to shift the direction of Russian economic development, reducing the power of big business and increasing the power of the state.
The ex-KGB men in the Kremlin may also be unhappy about Mr Khodorkovsky's reported willingness to sell a controlling stake in Yukos to the US oil major, Exxon-Mobil.
Yukos became one of the world's biggest oil companies this year, when it merged with Sibneft - it was just before this merger took effect that one of Mr Khodorkovsky's closest associates, Yukos general director Platon Lebedev, was arrested. (He is still in jail, under investigation.)
The idea that this Russian giant could be consumed by a US giant causes alarm in some circles.
"Yukos is offering itself as a Trojan horse to allow American interests to control Russian interests," wrote one concerned Russian newspaper.
Another observer noted that "foreigners" would not be likely to contribute funds to President Putin's election campaigns.
More generally, Yukos has been leading a trend in Russian business towards increased openness and transparency - introducing Western rules to Russia.
This too is not in the interests of Kremlin officials, who have more influence when the game is played under the old Russian system, with its traditions of secrecy and backroom deals.
Where Mr Putin himself stands in the Yukos affair is hard to establish.
A big supporter of the modernisation of the Russian economy, and an enthusiast for foreign investment, he has been unusually quiet about the company's problems.
He has either chosen to take a back seat - adopting a wait-and-see approach as his backers fight among themselves - or he has been pushed to one side by powerful forces at work behind the scenes.