The study of physical activity behavior in youth generally lacks a sufficient theoretical foundation for examining variables that influence that behavior. This is a major limitation because theory guides the search for determinants of behavior and the subsequent interplay between research findings and application. Theory offers a systematically organized body of knowledge that is applicable in a relatively wide variety of circumstances. This body of knowledge provides assumptions, accepted principles, and rules that provide a blueprint for analyzing, predicting, or explaining behavior. Hence, theory is essential for well-formulated examinations of physical activity behavior. This chapter describes two well-established theories for understanding physical activity behavior, namely, social cognitive theory and self-determination theory. The description provides a overview of major or primary tenets of the two theories; full elaborations of social cognitive theory and self-determination theory have been provided, for example, by Bandura (1986, 1997) and Deci and Ryan (1985, 2000), respectively.