The key figure behind the theory of psychoanalysis is Sigmund Freud. In the late 19th and early 20th centuries, Freud developed the concept of the unconscious mind and its influence on human behavior. His innovative techniques, such as free association and dream interpretation, helped unravel the complexities of emotional and psychological disturbances. Freud's work opened new pathways for understanding mental health and laid foundational theories that continue to shape psychology today. His legacy remains influential, sparking both admiration and criticism within the field.