About 2.5% of infants born in Finland are twins. Twins have long been objects for genetic studies, which nowadays have large study groups, e.g. twin registers from a whole country. Twins can also be studied from another point of view: the special situation of being a twin, and its consequences on development and mental health. Perinatal mortality and morbidity are higher in twins than in singletons, and accordingly cumulative incidences of various handicaps are higher in twins. The human relationships of twins have their special features from the early beginning. Twins have to share the attention from the parents, and some parents resolve the situation by sharing the twins: "mother's child" and "father's child" may develop. Twins may be dependent on each other, and the inter-twin relationship can also be characterized by dominance-submissiveness. In adolescence, the time of getting independence from the parents, twins have also another task of development: they have to grow up from the co-twin dependence in order to become autonomous adults. In comparison to much bigger twin materials of genetic studies, follow-up studies in birth cohorts have their benefits, too. They give us a good opportunity to research the development of human relationships in twin families and their consequences on both somatic and mental health.