Symbiosis in protozoa mostly represents a close mutualistic association between a protozoan and unicellular symbionts (bacteria, cyanobacteria or/and unicellular algae) or protozoans and a multicellular organism (ruminants, lower termites, wood-eating cockroaches, plants). Benefits for a protozoan host can be: extra food supply from intracellular algae or cyanobacteria, protection from predation and oxygen supply. Benefits for symbionts can be: protection from predation and nutrition supply. One protozoan can bear a couple of hundreds unicellular symbionts. Symbiosis can be obligate or facultative, endosymbiosis or ectosymbiosis. It is very common in ciliate and sarcodine protozoans. The best studied symbioses are those between a ciliate protozoan of genus Paramecium and symbiotic unicellular algae of genus Chlorella. This work presents a short overview of the most common examples of symbiosis in protozoa. It shows the necessity of adaptations of symbionts and hosts for formation and maintainance of the symbiosis. Symbiosis is very important because it allows these organisms to take new place in ecological niche which is out of reach to other organisms. Symbiosis in protozoa in nature is very common and shows a great diversity, but is still not adequately investigated.