What astrophysical sources accelerate cosmic rays? This nearly 100-year-old question is a primary objective for the field of high-energy particle astrophysics. This question is not just important because of its age, but also because of the broad impact of cosmic rays on many scientific fields. Cosmic rays have lead to, and may in the future lead to, a new understanding of particle physics. The accelerators of cosmic rays produce particles of energies far exceeding human capabilities. Black holes and intense gravitational and electromagnetic fields power these accelerators providing unique laboratories that cannot be replicated on Earth. Cosmic rays propagate throughout the Universe and serve as unique probes of the distant universe and dark matter. Cosmic rays influence and probe the dynamics of our Galaxy, where their energy density is comparable to that of starlight and electromagnetic fields. Outside of Physics and Astronomy, Biologists and climate scientists have even explored the effects of cosmic rays on evolution and the Earth's climate. The study of cosmic rays is truly multi-disciplinary.
High-energy gamma-ray observations are an essential probe of cosmic rays, because gamma rays are created by cosmic rays interacting near their origin. The resulting gamma rays travel in straight lines, unperturbed by the Galactic and extra-Galactic magnetic fields, and, unlike the charged cosmic rays, point back to their sources, providing the direction to the cosmic ray accelerator. In addition, the characteristics of the gamma-ray flux variability and spectra constrain the acceleration mechanisms and the environment of the accelerator. The highest energy gamma rays and the shortest timescales of variability provide the strongest constraints on the acceleration mechanisms at work in these sources. These two objectives high energy and transient observations are the primary scientific motivation for HAWC and are described here. In addition, HAWC's wide field of view provides a unique discovery potential. History has shown that astronomical surveys of new wavebands produce unexpected and amazing observations.