This contribution re-evaluates the theoretical foundations of Kleinian psychoanalysis. Both the resistance to her work outside England and the relevance it has for working with children and psychotic patients are considered. Klein stresses the crucial importance of constitutional elements in psychic development and consequently focuses on internal dispositions and fantasies. The concepts of death-drive, continuity between normality and pathology, projective identification, and unconscious fantasy are explained and illustrated with clinical material. Finally their consequences for our diagnostic and therapeutic thinking are considered.