  The Serendipitous Extragalactic X-ray Source Identification (SEXSI) Program
is designed to expand significantly the sample of identified extragalactic hard
X-ray sources at intermediate fluxes, 1e-13 erg/cm2/s < S_(2-10 keV) < 1e-15
erg/cm2/s. SEXSI, which includes sources derived from more than 2 sq deg of
Chandra images, provides the largest hard X-ray-selected sample yet studied,
offering an essential complement to the Chandra Deep Fields (total area ~0.2 sq
deg). In this paper we describe R-band optical imaging of the SEXSI fields from
the Palomar, MDM, and Keck observatories. We have identified counterparts, or
derived flux limits for nearly 1000 hard X-ray sources. Using the optical
images, we derive accurate source positions. We investigate correlations
between optical and X-ray flux, and optical flux and X-ray hardness ratio. We
also study the density of optical sources surrounding X-ray counterparts, as
well as the properties of optically-faint, hard X-ray sources.
