The thesis shows the complexity of identity as a concept, and critically examines some of it?s fields of applications. The research conducts an historical analysis of the concept as it is used in the disciplines of sociology, philosophy, social psychology, psychoanalysis, educational theory, ethnology and anthropology. An identity theory is developed within the framework of a social action theory that understands identity as a phenomenon which is embedded in several inter-related contexts (biopsychosocial, social, ethical, geographic-ecological, temporal context). The aim of the present investigation is not only theoretical; rather it is a theoretically initiated empirical analysis of the concept of identity. Some basic dimensions of the concept (colloquial meaning of the concept, self-definition, self-consciousness) are identified by a short questionnaire given to a random sample of 173 people, aged 14 to 82, using some open questions. This is followed by 38 qualitative interviews in which the temporal context of identity is analyzed. The development of identity is investigated in the course of biography.Social identity is examined using quantitative methods. The empirical analysis is based on the module "National Identity II" of the International Social Survey Programme (ISSP), which was conducted in 33 countries (N = 42.280), between the years 2003 and 2005. Using multilevel-regression analyses the following research question is examined: Do differences in the social structure of the different nations cause differences in the structure of people?s identities. The results show clearly that the context of a country greatly influences the subjective assessment of the participants? social identity. Finally, it is proven empirically that the concept of identity is able to explain a considerable amount about the variances in attitudes, for example, life satisfaction.