Abstract: The High Altitude Water Cherenkov (HAWC) experiment, under construction at Sierra Negra, Mexico, consists of a 22500 square meter area of water tanks instrumented with light-sensitive photo-multiplier tubes. The experiment detects energetic secondary particles reaching the ground when a high-energy cosmic ray or gamma ray interacts in the atmosphere above the experiment. High-energy gamma ray astronomy provides a probe of some of the most gravitationally and electromagnetically extreme regions in the universe, from neutron stars and supernova remnants to active galactic nuclei and gamma-ray bursts. High-energy gamma ray are also key to understanding the origin of galactic cosmic radiation. I will describe the design of the HAWC instrument, scheduled to be completed in 2014, and discuss the motivation and scientific return the experiment will bring.