COSI is a Compton telescope selected by NASA for the SMEX program (with launch in 2025). COSI will cover the “MeV Gap” providing a significant improvement in sensitivity along with high-resolution spectroscopy, enabling studies of 511 keV electron-positron annihilation emission and measurements of several radioactive elements that trace the Galactic history of supernovae. COSI also measures polarization of gamma-ray bursts (GRBs), AGNs, accreting galactic black holes, and pulsars as well as detecting and localizing multimessenger sources. We describe the COSI science, the instrument, and its capabilities.