  The hard x-ray (10-600 keV) sky is inherently time variable and yet has
neither been surveyed nor monitored with a sensitive imaging telescope. The
Energetic X-ray Imaging Survey Telescope (EXIST) is a mission concept, proposed
for MIDEX, which would conduct the first imaging all-sky hard x-ray survey as
well as provide a sensitive all sky monitor. With $\sim 60%$ sky coverage each
orbit, and full sky coverage each 50 days, hard x-ray studies of gamma-ray
bursts, AGN, galactic transients, x-ray binaries and accretion-powered pulsars
can be conducted over a wide range of timescales. We summarize the scientific
objectives of EXIST for both the survey and monitoring objectives. We describe
the mission concept and the instrumentation approach, which would incorporate a
large area array of Cd-Zn-Te (CZT) detectors, as well as some of our ongoing
development of CZT array detectors.

Astrophysics