Quantum field theory is central to current research in particle physics, condensed-matter systems, and early-universe cosmology. It describes the interactions of elementary particles, the dynamics of many-body systems, and the behaviour of "critical phenomena" with exquisite accuracy. String theory is based on the idea that, at very short distances, all particles extend into one-dimensional objects called “strings.” Modern string theory is a broad and varied field with strong connections to quantum gravity, particle physics, cosmology, and mathematics. Perimeter researchers are producing world-leading advances in quantum field theory and string theory.