SenatorJoe Biden used the quotation in the 2008 vice-presidential debate against Sarah Palin when he was accused of focusing too much on the past. The phrase is also commonly used by the military when discussing the similarities between war throughout history.
Although the phrase is now commonly used to mean what's stated above, as the phrase was originally used in The Tempest, Act 2, Scene I, it means that all that has happened before that time, the "past," has led Antonio and Sebastian to this opportunity to do what they are about to do, commit murder. In the context of the preceding and next lines, "(And by that destiny) to perform an act, Whereof what's past is prologue; what to come, In yours and my discharge" Antonio is in essence rationalizing to Sebastian, and the audience, that he and Sebastian are fated to act by all that has led up to that moment, the past has set the stage for their next act, as a prologue does in a play. Therefore, this phrase might be better used in situations where people are attempting to rationalize wicked acts based on the past. It can also be taken to mean that everything up until now has merely set the stage for Antonio and Sebastian to make their own destinies; in this context it does not indicate that their future acts are fated, but rather that everything up to that point was merely like a prologue, not the important story.
The phrase is the name of the musician Tycho's second album.
A slightly modified part of the phrase is used as the title of the comic book Robotech Zero : What's Past is Prelude by Academy Comics.
The phrase was used in a commercial for the video game Metroid Other M, indicating it was a prequel to the 2003 title Metroid Fusion because the events of the game explained what happened to Adam Malkovich, Samus Aran's former CO.
"What's Past, Is Prologue" is a title from the Built on Secrets' album "Reflection".