Austrian-born British psychoanalyst known for her work with young children, in which observations of free play provided insights into the child's unconscious fantasy life, enabling her to psychoanalyze children as young as two or three years of age.

The youngest child of a Viennese dental surgeon, Klein expressed an early interest in medicine but abandoned her plans when she married at 21. The marriage, though unhappy, produced three children. She became interested in psychoanalysis in Budapest a few years before World War I, undergoing psychoanalysis with Sándor Ferenczi (Ferenczi, Sándor), himself a close associate of Freud. Ferenczi urged her to study the psychoanalysis of young children, and in 1919 she produced her first paper in the field. Two years later she was invited by Karl Abraham to join the Berlin Psychoanalytic Institute, remaining there until 1926, when she moved to London.

In The Psychoanalysis of Children (1932), she presented her observations and theory of child analysis. Believing children's play to be a symbolic way of controlling anxiety, she observed free play with toys as a means of determining the psychological impulses and ideas associated with the early years of life. Her object-relations theory related ego development during this period to the experience of various drive objects, physical objects that were associated with psychic drives. In early development, she found, a child relates to parts rather than to complete objects—for example, to the breast rather than to the mother. This unstable and primitive mode of identification was termed by Klein the paranoid-schizoid position. The next development phase is the depressive position, in which the infant comes to relate to whole objects, such as the mother or father. This phase is marked by the infant's recognition of the ambivalence of his feelings toward objects, and thus the moderation of his internal conflicts about them.

Klein believed that the anxiety in the paranoid-schizoid position was persecutory, threatening the annihilation of the self, and the anxiety of the second, later position was depressive, being related to fear of the harm done to loved objects by the infant's own destructive impulses.

Beginning in 1934 Klein used her work with adult patients to clarify and extend her ideas on infant and childhood anxiety, presenting her views in a number of papers and a book, Envy and Gratitude (1957). Her final work, published posthumously in 1961, Narrative of a Child Analysis, was based on detailed notes taken during 1941.