EXIST was first proposed in 1995 and was recommended as a high priority mission for the coming decade by the NASA Gamma Ray Program Working Group (GRAPWG), as summarized in the 1999 GRAPWG Report (pdf, page 34). The 2001 Decadal Survey Report recommended EXIST as a mission for this decade. EXIST was included in the 2000 NASA Strategic Plan as a Mid-Term mission, and would serve as the Black Hole Finder Probe in the Beyond Einstein Program as described in the current (2003) Structure and Evolution of the Universe Roadmap. A new Universe Exploration Roadmap is currently being developed and will be released in mid 2005.
EXIST was originally designed to be attached to the International Space Station (ISS), but was redesigned in 2001 as a free-flying mission. The design was further refined in Fall 2004, notably by adding a low-energy telescope to improve source position determinations and extend the energy coverage.
The new design is to keep the instrument spec in 2004 design and to reduce the burden, for the S/C, arising from the asymmetric launching configuration.