A set of concepts and propositions that describe, organize, and explain a set of observations. What makes a theory good? must be the least complex explanation must be able to be tested must predict future behaviour. Contributions: highlight the importance of biological as a basis for development. Criticisms: not nearly enough attention paid to environment. Psychoanalytic theories: conflict of individual"s instinct and societal norms for behaviour. Stages propose shifts in focus on parts of body. Viewed children as more active in development than freud. More emphasis on social and cultural influences on development. Criticisms idea of unconscious motivation focus on later consequences of early experiences no real evidence of early conflicts affecting adult personality. Only overt behaviours should be measured and analyzed. Strong emphasis on environmental influences recall locke"s tabula rasa. Applications of watson"s behaviourism: f. skinner"s radical behaviourism. Focus on outcome of behaviour for predicting future occurrences of that behaviour. Reinforcers probability of behaviour occurring again.