A lot of speculation at the moment, pending the outcome of the post mortem, but the police have said sleeping pills were found near the body and they suspect an overdose.
Perhaps the most telling comments are those the actor gave in an interview before Christmas:
"Ledger was taking sleeping pills after completing I'm Not There, in which he played one of the many incarnations of singer Bob Dylan.
He told The New York Times in a November interview that he "stressed out a little too much'' during the Dylan film, and had trouble sleeping while portraying the Joker, whom he called a "psychopathic, mass-murdering, schizophrenic clown with zero empathy".
"Last week I probably slept an average of two hours a night," Ledger told the Times.
"I couldn't stop thinking. My body was exhausted, and my mind was still going."
He said he took two Ambien pills, which only worked for an hour, the paper said."