The advent of a next linear e{sup {+-}}e− collider and back-scattered laser beams will allow the study of a vast array of high energy processes of the Standard Model through the fusion of real and virtual photons and other gauge bosons. As examples, The author discusses virtual photon scattering [gamma]{sup *}[gamma]{sup *} → X in the region dominated by BFKL hard Pomeron exchange and report the predicted cross sections at present and future e{sup {+-}}e− colliders. The authors also discusses exclusive [gamma][gamma] reactions in QCD as a measure of hadron distribution amplitudes and a new method for measuring the anomalous magnetic and quadrupole moments of the W and Z gauge bosons to high precision in polarized electron-photon collisions.

The collisions of high energy photons produced at a electron-positron collider provide a comprehensive laboratory for testing QCD, electroweak interactions and extensions of the standard model. The luminosity and energy of the colliding photons produced by back-scattering laser beams is expected to be comparable to that of the primary e[sup +]e[sup [minus]] collisions. In this overview, we shall focus on tests of electroweak theory in photon-photon annihilation, particularly [gamma][gamma] [yields] W[sup +]W[sup [minus]], [gamma][gamma] [yields] Higgs bosons, and higher-order loop processes, such as [gamma][gamma] [yields] [gamma][gamma], Z[gamma] and ZZ. Since each photon can be resolved into a W[sup +]W[sup minus] pair, high energy photon-photon collisions can also provide a remarkably background-free laboratory for studying WW collisions and annihilation. We also review high energy [gamma][gamma] tests of quantum chromodynamics, such as the scaling of the photon structure function, t[bar t] production, mini-jet processes, and diffractive reactions.

The collisions of photons at a high energy electron-positron collider provide a comprehensive laboratory for testing QCD, electroweak interactions, and extensions of the standard model. It is expected that by using back-scattered laser beams that the effective luminosity and energy of photon-photon collisions will be comparable to that of the primary ee− collisions. In this talk, I will focus on tests of electroweak theory in photon-photon annihilation such as [gamma][gamma] 2!W+W−, [gamma][gamma] 2!Higgs boson, and higher-order loop processes, such as [gamma][gamma] 2![gamma][gamma], Z[gamma] and ZZ. Since each photon can be resolved into a W+W− pair, high energy photon-photon collisions can also provide a remarkable background-free laboratory for studying WW collisions and annihilation. I also review high energy [gamma][gamma] tests of quantum chromodynamics, such as the scaling of the photon structure function, t{bar t} production, mini-jet processes, and diffractive reactions.

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.

Research on photon and electron collisions with atomic and molecular targets and their ions has seen a rapid increase in interest, both experimentally and theoretically, in recent years. This is partly because these processes provide an ideal means of investigating the dynamics of many particle systems at a fundamental level and partly because their detailed understanding is required in many other fields, particularly astrophysics, plasma physics and controlled thermonuclear fusion, laser physics, atmospheric processes, isotope separation, radiation physics and chemistry and surface science. In recent years a number of important advances have been made, both on the experimental side and on the theoretical side. On the experimental side these include absolute measurements of cross sections, experiments using coincidence techniques, the use of polarised beams and targets, the development of very high energy resolution electron beams, the use of synchrotron radiation sources and ion storage rings, the study of laser assisted atomic collisions, the interaction of super-intense lasers with atoms and molecules and the increasing number of studies using positron beams.