Sigmund Freud's work revolutionized psychology through his development of the psychoanalytic theory. He introduced concepts such as the unconscious mind, defense mechanisms, and psychosexual stages of development. Freud emphasized the importance of early childhood experiences and their lasting effects on personality and behavior. His innovative techniques, particularly dream analysis and free association, provided new insights into human personality and mental processes. Despite controversies, Freud's ideas laid the foundation for various modern therapeutic practices and sparked further exploration into the realms of human psyche and mental health.