  The Soft X-ray Imager (SXI) is one of the three instruments on board EXIST, a
multi-wavelength observatory in charge of performing a global survey of the sky
in hard X-rays searching for Super-massive Black Holes (Grindlay & Natalucci,
these Proceedings). One of the primary objectives of EXIST is also to study
with unprecedented sensitivity the most unknown high energy sources in the
Universe, like high redshift GRBs, which will be pointed promptly by the
Spacecraft by autonomous trigger based on hard X-ray localization on board. The
presence of a soft X-ray telescope with an effective area of about 950cm2 in
the energy band 0.2-3 keV and extended response up to 10 keV will allow to make
broadband studies from 0.1 to 600 keV. In particular, investigations of the
spectra components and states of AGNs and monitoring of variability of sources,
study of the prompt and afterglow emission of GRBs since the early phases,
which will help to constrain the emission models and finally, help the
identification of sources in the EXIST hard X-ray survey and the
characterization of the transient events detected. SXI will also perform
surveys: a scanning survey with sky coverage 2pi and a limiting flux of
5x10^(-14) cgs plus other serendipitous.
