Presently, our understanding of nature is split into two domains. One theory applies to quantum phenomena, and is relevant on the small scale. A very different theory applies to gravity, space, and time, and is important for large-scale phenomena. Quantum gravity seeks to unify these into one single description of nature, based on a single set of principles. Perimeter researchers are actively pursuing a number of approaches to this challenge, including loop quantum gravity, spin foam models, string theory, causal set theory, shape dynamics, twistor theory, and asymptotic safety. They also seek to invent experiments that could test ideas about quantum gravity. The search for quantum gravity overlaps with other areas at PI such as cosmology, particle physics, and the foundations of quantum theory.