This chapter outlines some well-known theories of adolescence, as well as reviewing the results of some of the major research studies. While adolescence is quite clearly affected by social and economic factors, and may manifest itself differently depending on the cultural and historical context, some form of transitional stage is common to most societies. Irrespective of when the stage ends, however, in most western countries there are particular characteristics of adolescence that are of general relevance. There is general agreement by all who have written about adolescence that it makes sense to describe the stage as being one of transition. The psychoanalytic view of adolescence takes as its starting-point the upsurge of instincts which is said to occur as a result of puberty. The sociological or social-psychological approach to adolescence is marked by a concern with roles and role change, and with the processes of socialization.