  The recent hard X-ray surveys performed by INTEGRAL and Swift have started to
reveal the demographics of compact sources including Super-Massive Black Holes
hosted in AGNs and have proven invaluable in tracking explosive events as the
death of massive stars revealed by Gamma-Ray Bursts up to cosmological
distances. Whereas the observations have contributed significantly to our
understanding of the sources populations in the Local Universe, it has also
become evident that revealing the processes that drive the birth and evolution
of the first massive stars and galaxies would have required a further big step
in both sensitivity and capability to study transient phenomena since their
very beginning and covering different wavebands simultaneously. Therefore,
after its decennial history as a proposed hard X-ray survey mission, EXIST has
now turned into a new, more advanced concept with three instruments on board
covering the IR/optical and X-ray/soft gamma-ray bands. The EXIST new design
(Grindlay 2009a) is therefore much improved in its capability for prompt study
of GRBs (with autonomous determination of the redshift for many of them) and
broadband spectral studies of SMBHs and transients in the high energy band from
0.1 to several hundred keV, with sensitive optical/NIR and soft X-ray
identifications and followup studies.
