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