  Fundamental principles of local quantum field theory or of quantum gravity
can enforce consistency requirements on the space of consistent low-energy
effective field theories. We survey the various techniques that have been used
to put UV constraints on IR physics, including those from causality
considerations in the form of S-matrix positivity and bootstrap bounds,
scattering time delays, conformal field theory and holographic methods,
together with those that arise from landscape/swampland criteria such as the
weak gravity conjecture. We review recent applications of these constraints to
corrections to Standard Model physics, corrections to Einstein gravity, and
cosmological theories and highlight promising future directions.
