The complexity of human behaviour has intrigued mankind because of its conscious and subconscious motivations. Psychologists try to find the causes for these complexities and reasons of individual differences. The best evolved definition of psychology is that it is a scientific study of behaviour and mental processes. To elaborate, psychology as a science uses systematic and objective scientific methods to study overt (observable) behaviour like facial expressions, gestures, endocrine reactions and such, and covert (hidden/unobservable) behaviour or cognitive or mental processes like thinking, remembering, feelings and so on. This chapter summarizes the fundamental theories of psychology to assist in understanding the chapters that follow.