Gusev crater, previously interpreted as the depocenter for the Gusev-Ma'adim Vallis fluvio-lacustrine system, is a proposed landing site for one of the Mars Exploration Rovers (MER). Here we use new remote-sensing data from the Thermal Emission Imaging System (THEMIS) supplemented by data from the Thermal Emission Spectrometer (TES), Mars Orbiter Camera (MOC), and Mars Orbiter Laser Altimeter (MOLA) to characterize the geology of Gusev crater. Thermal infrared data from THEMIS and TES were used to map thermophysical units on the basis of relative albedos and diurnal temperature variations. THEMIS and MOC visible images were used to map unit morphologies and to estimate crater density ages. MOLA data were used to identify unit contacts and stratigraphic relationships. Various data were then combined to construct a new surface unit map and stratigraphy for units on the floor of Gusev. Seven surface units were identified in Gusev, mostly Hesperian in age, but with two showing evidence of later modification and redistribution. Five or more surface units and layering are present within the MER-A landing ellipse, attesting to the geologic diversity of this site. Surface units show features that could be consistent with fluvio-lacustrine, aeolian, and/or volcanoclastic deposition, but the spatial resolution of visible/infrared data does not allow for the identification of unambiguous volcanic or fluvio-lacustrine textures. However, a MER landing in Gusev may provide the opportunity to analyze multiple units, distinguish rock types, examine stratigraphic relationships, and shed light on the ancient depositional environment.