Have a 50 mCrab sensitivity at 5σ in a one-year survey. HAWC will observe hard-spectrum Galactic sources at TeV energies with a sensitivity similar to that of Fermi Gamma-ray Space Telescope at GeV energies, detect diffuse emission from regions of the Galactic plane, have sensitivity to see known TeV active galactic nuclei (AGN) and the brightest known GeV gamma-ray bursts (GRBs), and represents a large enough step in sensitivity to likely discover new phenomena. Measure the spectrum and spatially characterize the diffuse TeV emission from the Milky Way galaxy to probe the cosmic ray flux in other regions of the galaxy. Observe extragalactic transient sources, such as GRBs and AGN, and notify other observers promptly so they can make multi-wavelength and multi-messenger observations. Perform a deep, unbiased survey of the TeV gamma-ray and cosmic-ray sky to understand TeV astrophysical sources sufficiently to search for new fundamental physics effects. Have a 2 steradian (sr) instantaneous field of view to allow observations of diffuse gamma-ray emission from the plane of the Galaxy over a broad range of Galactic longitudes reaching to the Galactic center. This wide field of view also enables HAWC to observe phenomena such as GRBs, which are rare, from unknown directions, and last only a few seconds. HAWC can thus discover new TeV sources and observe flaring in known sources that may have no low energy counterpart, i.e. orphan TeV flares from AGN that are signatures of hadronic acceleration. Operate for at least five years with >90% duty cycle, which will give it sufficient exposure to measure the low fluxes at higher energies and long enough to detect and monitor a variety of transient sources. Have a median energy below 1 TeV for a Crab-like spectrum, which is needed to observe extragalactic sources that are attenuated at high energies by pair production with intergalactic photons. Have a >95% hadronic background rejection for E >10 TeV by distinguishing the penetrating particles in the hadron-initiated background showers from the gamma-ray initiated electromagnetic showers. Have an angular resolution of <0.5o for E >1 TeV and 0.25o for E >10 TeV. This resolution improves HAWC's flux sensitivity by rejecting the isotropic background and provides source localizations that are sufficient for targeting by other detectors and for determining the spatial morphology of the source. HAWC may also discover extended sources that can trigger deep observations by IACTs. Science goals Galactic sources at high energies