The Dark Energy Spectroscopic Instrument (DESI) is being installed at the Mayall 4-meter to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. DESI 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.