Psychoanalysis is a set of theories and therapeutic techniques that deal in part with the unconscious mind, and which together form a method of treatment for mental disorders. The discipline was established in the early 1890s by Sigmund Freud, whose work stemmed partly from the clinical work of Josef Breuer and others. Freud developed and refined the theory and practice of psychoanalysis until his death in 1939. In an encyclopedia article, he identified the cornerstones of psychoanalysis as – "the assumption that there are unconscious mental processes, the recognition of the theory of repression and resistance, the appreciation of the importance of sexuality and of the Oedipus complex." Freud's students Alfred Adler and Carl Gustav Jung developed offshoots of psychoanalysis which they called individual psychology (Adler) and Analytical Psychology (Jung), although Freud himself wrote a number of criticisms of them and emphatically denied that they were forms of psychoanalysis. Psychoanalysis was later developed in different directions by neo-Freudian thinkers, such as Erich Fromm, Karen Horney, and Harry Stack Sullivan.

Freud distinguished between a conscious and unconscious mind, arguing that the unconscious mind largely determines behaviour and cognition owing to unconscious drives. Freud observed that attempts to bring such drives into awareness triggers resistance in the form of defense mechanisms, particularly repression, and that conflicts between conscious and unconscious material can result in mental disturbances. He also postulated that unconscious material can be found in dreams and unintentional acts, including mannerisms and Freudian slips. Psychoanalytic therapy developed as a means to improve mental health by bringing unconscious material into consciousness. Psychoanalysts place a large emphasis on early childhood in an individual's development. During therapy, a psychoanalyst aims to induce transference, whereby patients relive their infantile conflicts by projecting onto the analyst feelings of love, dependence and anger.

During psychoanalytic sessions a patient traditionally lies on a couch, and an analyst sits just behind and out of sight. The patient expresses their thoughts, including free associations, fantasies, and dreams, from which the analyst infers the unconscious conflicts causing the patient's symptoms and character problems. Through the analysis of these conflicts, which includes interpreting the transference and countertransference (the analyst's feelings for the patient), the analyst confronts the patient's pathological defence mechanisms to help the patient understand themselves better.

Psychoanalysis is a controversial discipline, and its effectiveness as a treatment has been contested, although it retains influence within psychiatry. Psychoanalytic concepts are also widely used outside the therapeutic arena, in areas such as psychoanalytic literary criticism and film criticism, analysis of fairy tales, philosophical perspectives such as Freudo-Marxism, and other cultural phenomena.