  The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a drift scan
radio telescope operating across the 400-800 MHz band. CHIME is located at the
Dominion Radio Astrophysical Observatory near Penticton, BC Canada. The
instrument is designed to map neutral hydrogen over the redshift range 0.8 to
2.5 to constrain the expansion history of the Universe. This goal drives the
design features of the instrument. CHIME consists of four parallel cylindrical
reflectors, oriented north-south, each 100 m $\times$ 20 m and outfitted with a
256 element dual-polarization linear feed array. CHIME observes a two degree
wide stripe covering the entire meridian at any given moment, observing 3/4 of
the sky every day due to Earth rotation. An FX correlator utilizes FPGAs and
GPUs to digitize and correlate the signals, with different correlation products
generated for cosmological, fast radio burst, pulsar, VLBI, and 21 cm absorber
backends. For the cosmology backend, the $N_\mathrm{feed}^2$ correlation matrix
is formed for 1024 frequency channels across the band every 31 ms. A data
receiver system applies calibration and flagging and, for our primary
cosmological data product, stacks redundant baselines and integrates for 10 s.
We present an overview of the instrument, its performance metrics based on the
first three years of science data, and we describe the current progress in
characterizing CHIME's primary beam response. We also present maps of the sky
derived from CHIME data; we are using versions of these maps for a cosmological
stacking analysis as well as for investigation of Galactic foregrounds.
