Photon 2009: Summary of Theory Talks

The conference 'Photon 2009' on the structure and the interactions of the photon included sessions on photon-photon collisions and a future high-energy photon linear collider. This summary of theoretical contributions to the conference therefore has two parts. I will discuss the physics potential of photon colliders with an emphasis on the study of electroweak physics and the search for physics beyond the standard model. Secondly, I will describe a few highlights in recent progress in the understanding of the properties and the interaction of the photon, comprising the production of prompt photons, the photon structure and exclusive hadron production, small-x and total cross sections of deep inelastic scattering. Finally, I will review the status of the comparison of measurement and theory for the muon anomalous magnetic moment g-2.