Ledger is probably best known for starring in 2005's "Brokeback Mountain," about two cowboys who fall in love. He earned Oscar, Golden Globe and Screen Actors Guild nominations for that role.
Later this year, Ledger is slated to appear as the Joker in "The Dark Knight," the latest film in the Batman franchise. In a November interview with The New York Times, he revealed how he'd thrown himself into the role, saying, "Last week, I probably slept an average of two hours a night. … I couldn't stop thinking. My body was exhausted, and my mind was still going.''
According to the newspaper, he was taking the sleep medication Ambien to get rest, but to no avail. And according to celebrity news service TMZ.com, Ledger was ill at the time of his death.
"We know he had pneumonia," managing editor Harvey Levin told ABC News' "Nightline."
There had been other reports of drug use, and some say his appearance had been slipping lately.
"Unkempt would be the word that would come to people's mind," said Us Weekly editor in chief Janice Min. "You know that could be the sign of an eccentric actor or maybe the sign of something amiss."
Ledger may be the second young actor to die from drug-related causes in 2008. On Jan. 15, actor Brad Renfro, 25, was found dead in his Los Angeles home. Though his cause of death has yet to be determined, he had a history of drug and alcohol abuse and had been arrested multiple times for drug possession.
Additional reporting contributed by Richard Esposito, Russell Goldman and The Associated Press.