The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed very large area coded aperture telescope array, incorporating 8m^2 of pixellated Cd-Zn-Te (CZT) detectors, to conduct a full-sky imaging and temporal hard x-ray (10-600 keV) survey each 95min orbit. With a sensitivity (5sigma, 1yr) of ~0.05mCrab (10-150 keV), it will extend the ROSAT soft x-ray (0.5-2.5keV) and proposed ROSITA medium x-ray (2-10 keV) surveys into the hard x-ray band and enable identification and study of sources ~10-20X fainter than with the ~15-100keV survey planned for the upcoming Swift mission. At ~100-600 keV, the ~1mCrab sensitivity is 300X that achieved in the only previous (HEAO-A4, non-imaging) all-sky survey. EXIST will address a broad range of key science objectives: from obscured AGN and surveys for black holes on all scales, which constrain the accretion history of the universe, to the highest sensitivity and resolution studies of gamma-ray bursts it will conduct as the Next Generation Gamma-Ray Burst mission. We summarize the science objectives and mission drivers, and the results of a mission design study for implementation as a free flyer mission, with Delta IV launch. Key issues affecting the telescope and detector design are discussed, and a summary of some of the current design concepts being studied in support of EXIST is presented for the wide-field but high resolution coded aperture imaging and very large area array of imaging CZT detectors. Overall mission design is summarized, and technology development needs and a development program are outlined which would enable the launch of EXIST by the end of the decade, as recommended by the NAS/NRC Decadal Survey.
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