The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The DESI instrument is designed to efficiently measure galaxy redshifts to z=1.6, quasars at all redshifts, and the 3-dimensional Lyman-alpha forest at z>2. In a 5-year survey to begin in 2019, DESI will map a total of 35 million galaxies and quasars over a survey footprint of 14,000 square degrees. I will review the design considerations for DESI, and for future, larger surveys.