This chapter examines how infant's experiences prompted by mastery motivation affect the construction of self-concept, and, conversely, how the development of self-concept affects the expression of mastery motivation. It begins with brief overviews of the constructs of self-concept and mastery motivation. Models are presented that graphically illustrate the interplay of self-concept and mastery motivation in development. Mastery motivation is the motivation to master the environment. This motivation is inferred when the apparent intent of an action or activity is to explore, influence or control the physical environment. Action theory has received relatively little attention in the field of mastery motivation, probably because it has focused on older children and adults, whereas the field of mastery motivation has focused on infants and very young children. Mastery motivation would be that part of the executive function that decides how much effort and resources to allocate to actions or tasks in which the goal is to master the environment.