  The Dark Energy Spectroscopic Instrument (DESI) has embarked on an ambitious
five-year survey to explore the nature of dark energy with spectroscopy of 40
million galaxies and quasars. DESI will determine precise redshifts and employ
the Baryon Acoustic Oscillation method to measure distances from the nearby
universe to z > 3.5, as well as measure the growth of structure and probe
potential modifications to general relativity. In this paper we describe the
significant instrumentation we developed for the DESI survey. The new
instrumentation includes a wide-field, 3.2-deg diameter prime-focus corrector
that focuses the light onto 5020 robotic fiber positioners on the 0.812 m
diameter, aspheric focal surface. The positioners and their fibers are divided
among ten wedge-shaped petals. Each petal is connected to one of ten
spectrographs via a contiguous, high-efficiency, nearly 50 m fiber cable
bundle. The ten spectrographs each use a pair of dichroics to split the light
into three channels that together record the light from 360 - 980 nm with a
resolution of 2000 to 5000. We describe the science requirements, technical
requirements on the instrumentation, and management of the project. DESI was
installed at the 4-m Mayall telescope at Kitt Peak, and we also describe the
facility upgrades to prepare for DESI and the installation and functional
verification process. DESI has achieved all of its performance goals, and the
DESI survey began in May 2021. Some performance highlights include RMS
positioner accuracy better than 0.1", SNR per \sqrt{\AA} > 0.5 for a z > 2
quasar with flux 0.28e-17 erg/s/cm^2/A at 380 nm in 4000s, and median SNR = 7
of the [OII] doublet at 8e-17 erg/s/cm^2 in a 1000s exposure for emission line
galaxies at z = 1.4 - 1.6. We conclude with highlights from the on-sky
validation and commissioning of the instrument, key successes, and lessons
learned. (abridged)
