BigBOSS is a stage IV dark energy survey. It will measure the spectroscopic redshifts of 20 million galaxies/quasars to z=3.5 and map the observable universe in 3D to half its edge. It will measure the cosmological distance and the expansion rate of the universe at multiple redshifts to sub-percent level accuracy through the baryon acoustic oscillation, measure the structure growth rate to percent level accuracy through the redshift space distortion and measure the 3D galaxy clustering to unprecedented accuracy. It will make major inroads in fundamental cosmological physics such as the nature of dark energy and gravity at cosmological scales, neutrino mass and the nature of inflation. It will also significantly advance many other areas such as galaxy formation and stellar astrophysics. The BigBOSS schedule currently proposes a construction start in 2014 and first light in 2018, followed by a five-year survey for the key cosmology project.