An increasing amount of data has revealed that many clusters are very complex systems. Optical analyses show that several clusters contain subsystems of galaxies, suggesting that they are still in the phase of dynamical relaxation. Indeed, there is a growing evidence that these subsystems arise as the consequence of group/cluster mergers. We here review the state of art of optical search and characterization of cluster substructures. We describe the effects cluster mergers have on optical measures of cluster dynamics, and on the properties of cluster member galaxies. We also discuss cluster mergers in relation to the large scale structure of the universe.
