  We describe the Sloan Digital Sky Survey IV (SDSS-IV), a project encompassing
three major spectroscopic programs. The Apache Point Observatory Galactic
Evolution Experiment 2 (APOGEE-2) is observing hundreds of thousands of Milky
Way stars at high resolution and high signal-to-noise ratio in the
near-infrared. The Mapping Nearby Galaxies at Apache Point Observatory (MaNGA)
survey is obtaining spatially-resolved spectroscopy for thousands of nearby
galaxies (median redshift of z = 0.03). The extended Baryon Oscillation
Spectroscopic Survey (eBOSS) is mapping the galaxy, quasar, and neutral gas
distributions between redshifts z = 0.6 and 3.5 to constrain cosmology using
baryon acoustic oscillations, redshift space distortions, and the shape of the
power spectrum. Within eBOSS, we are conducting two major subprograms: the
SPectroscopic IDentification of eROSITA Sources (SPIDERS), investigating X-ray
AGN and galaxies in X-ray clusters, and the Time Domain Spectroscopic Survey
(TDSS), obtaining spectra of variable sources. All programs use the 2.5-meter
Sloan Foundation Telescope at Apache Point Observatory; observations there
began in Summer 2014. APOGEE-2 also operates a second near-infrared
spectrograph at the 2.5-meter du Pont Telescope at Las Campanas Observatory,
with observations beginning in early 2017. Observations at both facilities are
scheduled to continue through 2020. In keeping with previous SDSS policy,
SDSS-IV provides regularly scheduled public data releases; the first one, Data
Release 13, was made available in July 2016.
