  The Compton Spectrometer and Imager (COSI) is a balloon-borne, soft-gamma ray
imager, spectrometer, and polarimeter with sensitivity from 0.2 to 5 MeV.
Utilizing a compact Compton telescope design with twelve cross-strip,
high-purity germanium detectors, COSI has three main science goals: study the
511 keV positron annihilation line from the Galactic plane, image diffuse
emission from stellar nuclear lines, and perform polarization studies of
gamma-ray bursts and other extreme astrophysical environments. COSI has just
completed a successful 46-day flight on NASA's new Super Pressure Balloon,
launched from Wanaka, New Zealand, in May 2016. We present an overview of the
instrument and the 2016 flight, and discuss COSI's main science goals,
predicted performance, and preliminary results.
