  Dark sectors, consisting of new, light, weakly-coupled particles that do not
interact with the known strong, weak, or electromagnetic forces, are a
particularly compelling possibility for new physics. Nature may contain
numerous dark sectors, each with their own beautiful structure, distinct
particles, and forces. This review summarizes the physics motivation for dark
sectors and the exciting opportunities for experimental exploration. It is the
summary of the Intensity Frontier subgroup "New, Light, Weakly-coupled
Particles" of the Community Summer Study 2013 (Snowmass). We discuss axions,
which solve the strong CP problem and are an excellent dark matter candidate,
and their generalization to axion-like particles. We also review dark photons
and other dark-sector particles, including sub-GeV dark matter, which are
theoretically natural, provide for dark matter candidates or new dark matter
interactions, and could resolve outstanding puzzles in particle and
astro-particle physics. In many cases, the exploration of dark sectors can
proceed with existing facilities and comparatively modest experiments. A rich,
diverse, and low-cost experimental program has been identified that has the
potential for one or more game-changing discoveries. These physics
opportunities should be vigorously pursued in the US and elsewhere.
