US comedian and actor Bill Cosby has gone on trial in Pennsylvania for sexual assault.
Former university employee Andrea Constand alleges he drugged and molested her in 2004.
Mr Cosby, 79, has said he is innocent and the encounter was consensual. The trial is expected to last about two weeks.
It is being seen as the biggest US celebrity court case since the murder trial of former American football player OJ Simpson in 1995.
Several of Mr Cosby's co-stars from the 1984-1992 sitcom The Cosby Show are expected to attend the trial in Norristown, outside Philadelphia. One of them, Keshia Knight Pulliam who played his daughter on TV, accompanied him into court.
His wife of 53 years was not seen with him as he entered the suburban courthouse.
More than 100 journalists are expected to cover the trial.
What is Cosby accused of?
Mr Cosby faces three counts of aggravated indecent assault. Each count carries a maximum sentence of 10 years and a fine of up to $US25,000.
In 2004 Ms Constand was 31 when she visited Mr Cosby's home seeking career advice after befriending him through Temple University in Philadelphia, where he served on the board of trustees.
She said Mr Cosby gave her three blue pills which made her legs feel "like jelly" and that he then began to touch her inappropriately.
Mr Cosby's defence lawyers argued on Monday that the pair had multiple romantic encounters, and that Mr Cosby only offered her drugs after she complained of trouble sleeping.
In 2006, the comedian settled with Ms Constand after providing an undisclosed cash sum to her.
At least 50 women have accused Mr Cosby of sexual assault, but he only faces charges in Ms Constand's case because of statutes of limitations.
What is the evidence against him?
Prosecutor Kristen Feden called upon jurors to look beyond his role as Dr Cliff Huxtable.
The wholesome TV image "will be shattered", she said, accusing Mr Cosby of "heinous crimes".
By drugging women and making them legally unable to consent to sex, Mr Cosby put them in a position "so that they couldn't say no", Mrs Feden told jurors.
She also frequently referred to Mr Cosby's own admission in 2005 that he had used drugs as part of his efforts to have sex with women.
The drugs, called Quaaludes, are a sedative that was widely used recreationally in the US in the 1970s.
However, Judge Steven O'Neill has ruled that the court will not hear about the 2006 civil case. Neither will jurors hear from Mr Cosby's many other accusers.
What is Mr Cosby's defence?
Mr Cosby said his encounter with Ms Constand was consensual, and his lawyer pointed to evenings spent sipping cognac at his home as proof of their friendship.
He also says he offered women drugs as a way to encourage them to relax rather than a way to render them helpless.
On the night of the alleged sex assault, Mr Cosby says he gave Ms Constand some Benadryl, an anti-allergy medication that can cause drowsiness, because she appeared stressed.
Mr Cosby's lawyers asked on Monday why Ms Constand returned to Mr Cosby's house after she said he had made previous unwanted sexual advances on her.
They also presented evidence that the two had more than 70 phone discussions after the alleged incident, and accused Ms Constand of changing her story to police at least three time.
They also questioned why the case is being brought now, more than 10 years after a previous district attorney decided there was not enough evidence to go to trial.
Will he testify?
Mr Cosby has said he will not testify and last month told Sirius-XM radio that he feared prosecution lawyers could twist his words on the stand.
"When you have to deal with examination, cross-examination, et cetera, et cetera, more than two sides to every story - sometimes it's four or five," he said.
He has also speculated that some of the allegations against him could be motivated by racism.
Why is Bill Cosby important?
Mr Cosby is best known for his role as the father in the television hit The Cosby Show, which was a huge hit in the US and around the globe.
At one point he was the highest-paid actor in the US.
He was born in Philadelphia and started his career as a stand-up comedian in 1962, debuting on NBC's The Tonight Show in 1963.
In 1965, he became the first black actor to star in a drama series in the US, in the espionage show I Spy. He earned three consecutive best actor Emmys for his role, at a time of high race tensions in the US.
In 2013 he did a national stand-up tour and received rave reviews. After that, several women began to come forward with allegations of sexual assault, some going back nearly 30 years.