Written by a methodological behaviorist, this treatise critiques neo-Hullian, Freudian, Eriksonian, and Piagetian theories and presents an ethological perspective on behavior and personality development. The critique is extended to cover social learning, cognitive-developmental, neo-Freudian, and Skinnerian theories, as well as the ideas of Bandura. Assumptions for conducting research and allowing for interpretation and integration of findings are specified and discussed. These assumptions include the following ideas: (1) that fundamentally important behaviors can be seen in conflict situations; (2) that all behavior must be viewed with the past history of the subject in mind; (3) that important manipulative skills can be noted directly and that covert behavior can be inferred by an observer who has been engaged in an extensive longitudinal study; (4) that only unobtrusive observations used to interpret the behavior of single subjects will be unbiased; (5) that all behavior is directly or indirectly interactive with the environment, is homeostatic, or works toward homeostasis and that all species-typical behavioral developments are adaptive; (6) that all behaviors must be construed "within the subject's perceptual-thought system"; and (7) that interpretation involves comparing present behaviors with similar past behaviors and interpreting them in terms of various possible types of behaviors and in terms of specified mechanisms of change. Any theory that fulfills these assumptions is considered to be an ethological theory of personality development. A bibliography of suggested readings is appended. (RH)