Dolphins (Delphinidae) are the most diversified family of all marine mammals. They belong to the order Cetacea. Many specific adaptations to aquatic life, a very developed brain and high intelligence single them out from other mammal species. The research of social evolution of dolphins and the comparison to land mammals brought contrary conclusions: one is convergence towards some land mammals, and the other is a social behaviour totally different from the rest of the animal world. Dolphins live in groups that, depending on species and on other factors, vary in size and duration. Life in a group has many advantages such as more successful hunting, defence from predators and protection of territory. It also provides greater possibility of social interactions which include mating, parental care and transfer of information and knowledge. Competition for food and sexual partners often leads to aggressive behaviour. Beside sexual intercourse, dolphins often have homosexual interactions in order to show dominance, or communion. Sexual behaviour of juveniles can be considered as a part of practice or just as a game. Dolphins have an exceptionally long period of parental care, in which they protect their infants from predators and teach them hunting techniques essential for independent life. They usually use acoustic communication, which can be vocal and non-vocal. In some species particularly developed forms of vocal communication were observed. Orcas have different dialects, bottlenose dolphins and some other species have signature whistles. Beside group life within one species, many dolphins were seen in company of other species. Because of their complex social relationships and sophisticated learning skills, dolphins are among the most intelligent and sociable animals.