  The Compton Spectrometer and Imager (COSI) is a 0.2-5 MeV Compton telescope
capable of imaging, spectroscopy, and polarimetry of astrophysical sources.
Such capabilities are made possible by COSI's germanium cross-strip detectors,
which provide high efficiency, high resolution spectroscopy and precise 3D
positioning of photon interactions. Science goals for COSI include studies of
0.511 MeV emission from antimatter annihilation in the Galaxy, mapping
radioactive elements from nucleosynthesis, determining emission mechanisms and
source geometries with polarization, and detecting and localizing
multimessenger sources. The instantaneous field of view (FOV) for the germanium
detectors is >25% of the sky, and they are surrounded on the sides and bottom
by active shields, providing background rejection as well as allowing for
detection of gamma-ray bursts or other gamma-ray flares over >50% of the sky.
We have completed a Phase A concept study to consider COSI as a Small Explorer
(SMEX) satellite mission, and here we discuss the advances COSI-SMEX provides
for astrophysics in the MeV bandpass.
