This paper offers a number of clinical studies of aggressive manifestations in children in latency and adolescence, and discusses the diagnostic considerations which lead to the differentiation of two dynamically separate groups of aggression. In the first group, the evidence of the immature ego's inability to deal with aggressive irruptions from the id is discussed in relation to considerations of general ego weakness, the state of object relations and of primitive defences. The author makes striking use of recent contributions to ego psychology, notably by Anna Freud, Beata Rank, and Hartmann, Kris, and Lowenstein. The second group covers disturbances at different levels of libidinal development resulting in unsatisfactory fusion with aggressive drives (sado-masochism). The paper is concerned to demonstrate the importance of the conception of aggression as an instinctual drive for the elucidation of these problems, and opposes the contention of Franz Alexander that aggression is mainly an adeptive phenomenon.