Abstract: It has been more than a century since the discovery of cosmic rays, and yet there remains much that we do not know about these mysterious particles. In particular, it is an open question as to where many of the cosmic rays come from and how they are accelerated to such incredible energies. The resolution to these puzzles will require not only measurements of the cosmic rays themselves, but also information provided by gamma-ray, X-ray, optical, infrared, and radio telescopes, gravitational wave detectors, and high-energy neutrino telescopes. It will be through this program of multi-messenger astronomy that we will finally be able to identify the sources of the cosmic rays and cosmic neutrinos, and begin to understand the physics behind these naturally occurring engines of high-energy particle acceleration. This observational program will also provide exciting opportunities for the field of particle physics, allowing us to test a variety of new physics scenarios, to search for the particle nature of dark matter, and to further probe the physics of neutrino oscillations.