Personality is defined as the dynamic organization, within an individual, of the systems that determine his or her characteristic behavior or thought. The study of personality involves many aspects of human behavior. Four approaches used to study personality are presented in this document: (1) Social Learning theory; (2) Trait theory; (3) Phenomenological theory; and (4) Cognitive-Development theory. Some principles of these theories overlap, but basic concepts are distinguishable. Social Learning theory's basic tenet is that human behavior results from an organism's past learning as it has developed through interaction with the environment. Trait theory asserts that personality is more or less organized by persistent characteristics or dimensions of individual differences--within each individual there exist predispositions to respond to situations in a certain manner. Phenomenological theory stresses the cognitive acquisition of meaning from stimuli an individual is presented with in life's daily activities. Cognitive-Developmental theory traces development of personality from birth to adulthood as capacities and concerns and thought processes develop. Some theorists posit that all normal individuals pass through the same sequential periods. Major theorists and approaches to personality assessment are described for each approach. Contains 23 references. (JBJ)