Basics of a photon collider
  This small review is devoted to $\gamma\gamma$ collisions including methods of creating the colliding $\gamma \gamma$ beams of high energy and physical problems which can be solved or clarified in such collisions. Contents:   1. Introduction 1.1. The subject 1.2. Interaction of photons in the Maxwell theory and QED 2. Collisions of equivalent photons at $e^+e^-$ storage rings 3. Results obtained in virtual $\gamma^*\gamma^*$ collisions 4. Linear $e^{\pm} e^-$ collider 5. Photon collider on a base of a linear $e^{\pm} e^-$ collider 5.1. Idea of high-energy $\gamma\gamma$ and $\gamma e$ colliders with real photons 5.2. Scheme of a photon collider 5.3. Compton scattering as a basic process for $e\to \gamma$ conversion 6. Physics of $\gamma\gamma$ interactions 7. Concluding remarks 7.1. Summary from TESLA TDR 7.2. Prediction of Andrew Sessler in 1998 7.3. Conclusion of Karl von Weizs\"acker for young physicists 
Valery G. Serbo (Novosibirsk State University)