I give a brief review of the history of photon--photon physics and a survey of its potential at future electron--positron colliders. Exclusive hadron production processes in photon--photon and electron--photon collisions provide important tests of QCD at the amplitude level, particularly as measures of hadron distribution amplitudes. There are also important high energy \gamma \gamma and e \gamma tests of quantum chromodynamics, including the production of jets in photon--photon collisions, deeply virtual Compton scattering on a photon target, and leading-twist single-spin asymmetries for a photon polarized normal to a production plane. Since photons couple directly to all fundamental fields carrying the electromagnetic current including leptons, quarks, W's, and supersymmetric particles, high energy \gamma \gamma collisions will provide a comprehensive laboratory for Higgs production and exploring virtually every aspect of the Standard Model and its extensions. High energy back-scattered laser beams will thus greatly extend the range of physics of the International Linear Collider.

PACS numbers: 12.38.--t, 13.66.Bc, 13.40.--f, 13.66.Lm