The Man Who Stepped into Yesterday (often abbreviated as TMWSIY) is a 1988 concept album written by Trey Anastasio, the guitarist and lead vocalist of the American rock band Phish, as his senior thesis while attending Goddard College.

Completed in July 1988, the thesis included an essay piece and collection of songs (recorded by Phish) relating an epic tale from the band's fictional land of Gamehendge.

Many other Phish songs also relate in some way to the Gamehendge saga, and appear on later albums and in live performances, including "Llama", "Punch You in the Eye" and "The Divided Sky".

[3] At one time Anastasio announced plans in Doniac Schvice, Phish's newsletter, to release the material as an interactive CD-ROM.

[5] This version of the suite does not include "Possum", but features "Llama", "McGrupp and the Watchful Hosemasters" and "Divided Sky", all of which are connected to the Gamehendge mythology or the story of Colonel Forbin, as well as "NO2", an experimental instrumental that was the B-side to the "Down with Disease" single and is used during the performance to soundtrack Anastasio's introductory narrative.