They didnt prove anything. They brought in 4 women with unproven hearsay accusations (he was never convicted of anything with these women) as "evidence" of a pattern to prove a case he had previously beat and was a mistrial.
Bill Cosby sex assault case: Seven questions answered
What was he on trial for?
He was on trial for a charge of three counts of felony indecent assault, alleged by a former colleague, Andrea Constand. Cosby has been found guilty on all three counts.
It is the only criminal case Cosby has faced thus far related to sexual assault accusations dating back years.
Ms Constand, a former employee of Temple University who now lives in Canada, has said that she came to think of Cosby as a "mentor and a friend" before she said he made unwanted sexual advances.
She went to visit him at his home in 2004 to discuss her career and her future where she said he drugged and sexually assaulted her.
A criminal complaint unsealed last year details the encounter.
She and Cosby settled a lawsuit in 2006 and she was paid an undisclosed sum.
Prosecutors re-opened her case last summer when they realised the statute of limitations had not run out and after other women had come forward.
There was a mistrial then a retrial. What changed?
The first trial failed to reach a verdict in the spring of 2017.
In the second trial, the judge allowed more evidence to be admitted, for the defence and the prosecution.
So the court heard testimony from four more accusers, evidence not allowed in the first trial, when only Ms Constand's account was heard.
That meant the prosecution could establish a pattern of behaviour on the part of the disgraced comedian.
There was also new evidence that worked in Cosby's favour. A new defence witness who once worked with Ms Constand had talked about fabricating evidence against a celebrity for financial gain.
And a $3.38m payment that Cosby paid to her in a 2006 settlement - which the defence said showed she was a gold-digger - had not been previously disclosed.