  We describe a new Large Program in progress on the Gemini North and South
telescopes: Gemini Observations of Galaxies in Rich Early Environments
(GOGREEN). This is an imaging and deep spectroscopic survey of 21 galaxy
systems at $1<z<1.5$, selected to span a factor $>10$ in halo mass. The
scientific objectives include measuring the role of environment in the
evolution of low-mass galaxies, and measuring the dynamics and stellar contents
of their host haloes. The targets are selected from the SpARCS, SPT, COSMOS and
SXDS surveys, to be the evolutionary counterparts of today's clusters and
groups. The new red-sensitive Hamamatsu detectors on GMOS, coupled with the
nod-and-shuffle sky subtraction, allow simultaneous wavelength coverage over
$\lambda\sim 0.6$--$1.05\mu$m, and this enables a homogeneous and statistically
complete redshift survey of galaxies of all types. The spectroscopic sample
targets galaxies with AB magnitudes $z^{\prime}<24.25$ and [3.6]$\mu$m$<22.5$,
and is therefore statistically complete for stellar masses
$M_\ast\gtrsim10^{10.3}M_\odot$, for all galaxy types and over the entire
redshift range. Deep, multiwavelength imaging has been acquired over larger
fields for most systems, spanning $u$ through $K$, in addition to deep IRAC
imaging at 3.6$\mu$m. The spectroscopy is $\sim 50$ per cent complete as of
semester 17A, and we anticipate a final sample of $\sim 500$ new cluster
members. Combined with existing spectroscopy on the brighter galaxies from
GCLASS, SPT and other sources, GOGREEN will be a large legacy cluster and field
galaxy sample at this redshift that spectroscopically covers a wide range in
stellar mass, halo mass, and clustercentric radius.
