Collective flow in heavy-ion collisions

We provide an overview of collective flow phenomena observed in heavy ion collisions from the Fermi energy range up to CERN Super Proton Synchrotron (SPS) energies. We summarize the experimental data in terms of the various observed aspects of flow, namely directed flow in the reaction plane, elliptic flow in-and out-of-plane, and azimuthally symmetric radial flow originating from the expansion of the hot and compressed reaction zone. Also reviewed are the theoretical concepts developed to simulate the complex reactions with the aim of extracting fundamental properties of hot and compressed nuclear matter.