  Low-energy tests of fundamental symmetries and studies of neutrino properties
provide a powerful window on physics beyond the Standard Model (BSM). In this
article, we provide a basic theoretical framework for a subsequent set of
articles that review the progress and opportunities in various aspects of the
low-energy program. We illustrate the physics reach of different low-energy
probes in terms of an effective BSM mass scale and illustrate how this reach
matches and, in some cases, even exceeds that accessible at the high energy
frontier.
