In Microscope, players use index cards to create the history of a fictional universe. The timeline of this history is built up non-chronologically, beginning with the broadest strokes of history and then narrowing down to specific events and scenes featuring characters from the fictional universe.
Play in Microscope takes place in rounds, with each round focusing on a specific lens or theme. Rounds have no fixed length, sessions can include an arbitrary number of rounds, and a game of Microscope can last for any number of sessions; a game of Microscope ends when the players agree to end it.
Microscope is a popular tool to create settings for other role-playing games, e.g. as part of session zero.
Microscope has one supplement, called Microscope Explorer. This supplement includes additional game mechanics for regular Microscope as well as three expansion or spin-off games called Union (in which players build a family tree instead of a timeline), Chronicle (which focuses on a single persistent thing), and Echo (which introduces time travel mechanics that can re-write history).