We review the present level of knowledge of the hadronic structure of the photon, as revealed in interactions involving quarks and gluons `in` the photon. The concept of photon structure functions is introduced in the description of deep-inelastic e gamma scattering, and existing parametrizations of the parton densities in the photon are reviewed. We then turn to hard gamma p and gamma gamma collisions, where we treat the production of jets, heavy quarks, hard (direct) photons, J/ psi mesons, and lepton pairs. We also comment on issues that go beyond perturbation theory, including recent attempts at a comprehensive description of both hard and soft gamma p and gamma gamma interactions. We conclude with a list of open problems.