Photon induced collisions: a phenomenologist's point of view

The analyses of HERA, LEP and Tevatron data involving photons in the initial and/or final states within perturbative QCD are reviewed. Specific features of parton distribution functions of the photon and their implications for proper definition of finite order QCD approximations are discussed. Particular attention is paid to theoretical uncertainties stemming from the ambiguities in the choice of renormalization and factorization scales. The importance of processes involving photons in initial or final states in searches for signals of new phenomena is emphasized and illustrated on the cases of pentaquarks and extra dimensions.