Sigmund Freud's theories revolutionized psychology by introducing concepts such as the unconscious mind, psychosexual development, and defense mechanisms. His pioneering psychoanalysis shifted the understanding of mental health, emphasizing the role of repressed desires and childhood experiences in adult behavior. Freud's methods, including dream analysis and talk therapy, provided new approaches for treating psychological disorders. His influence extended beyond psychology, affecting fields like literature, art, and philosophy, challenging prevailing notions of human behavior and motivating further exploration into the human psyche.