Building on the legacy of the Sloan Digital Sky Survey (SDSS-I and II),SDSS-III is a program of four spectroscopic surveys on three scientificthemes: dark energy and cosmological parameters, the history andstructure of the Milky Way, and the population of giant planets aroundother stars. In keeping with SDSS tradition, SDSS-III will provideregular public releases of all its data, beginning with SDSS DataRelease 8 (DR8), which was made public in 2011 January and includesSDSS-I and SDSS-II images and spectra reprocessed with the latestpipelines and calibrations produced for the SDSS-III investigations.This paper presents an overview of the four surveys that compriseSDSS-III. The Baryon Oscillation Spectroscopic Survey will measureredshifts of 1.5 million massive galaxies and Lyα forest spectraof 150,000 quasars, using the baryon acoustic oscillation feature oflarge-scale structure to obtain percent-level determinations of thedistance scale and Hubble expansion rate at z < 0.7 and at z ≈2.5. SEGUE-2, an already completed SDSS-III survey that is thecontinuation of the SDSS-II Sloan Extension for Galactic Understandingand Exploration (SEGUE), measured medium-resolution (R =λ/Δλ ≈ 1800) optical spectra of 118,000 stars ina variety of target categories, probing chemical evolution, stellarkinematics and substructure, and the mass profile of the dark matterhalo from the solar neighborhood to distances of 100 kpc. APOGEE, theApache Point Observatory Galactic Evolution Experiment, will obtainhigh-resolution (R ≈ 30,000), high signal-to-noise ratio (S/N >=100 per resolution element), H-band (1.51 μm < λ < 1.70μm) spectra of 105 evolved, late-type stars, measuringseparate abundances for ~15 elements per star and creating the firsthigh-precision spectroscopic survey of all Galactic stellar populations(bulge, bar, disks, halo) with a uniform set of stellar tracers andspectral diagnostics. The Multi-object APO Radial Velocity ExoplanetLarge-area Survey (MARVELS) will monitor radial velocities of more than8000 FGK stars with the sensitivity and cadence (10-40 m s-1,~24 visits per star) needed to detect giant planets with periods up totwo years, providing an unprecedented data set for understanding theformation and dynamical evolution of giant planet systems. As of 2011January, SDSS-III has obtained spectra of more than 240,000 galaxies,29,000 z >= 2.2 quasars, and 140,000 stars, including 74,000 velocitymeasurements of 2580 stars for MARVELS.