This chapter discusses the development of achievement motivation during adolescence from the perspective of expectancy-value theory, and explains how adolescents' expectancies for success and achievement values change during adolescence, particularly during educational transitions such as that from elementary to middle school and from middle to high school. In particular, the chapter focuses on the development of children's expectancies and values during adolescence, as well as gender differences in these beliefs and values. The chapter begins with consideration of some of the major biological, psychological, and environmental changes that occur at adolescence and then presents an expectancy-value model of achievement choice as a framework for understanding early adolescence and adolescents' performance and choice. Research findings on the development of children's competence beliefs, expectancies for success, and achievement values are discussed, focusing on changes in the structure of children's motivational beliefs and values, and on changes in the mean level of their beliefs and values. Discussion of gender differences highlights differences in beliefs about competence and achievement task values. The chapter also considers how educational transitions influence adolescents' expectancies and values, and how these expectancies and values influence the choice of activities that adolescents pursue, as well as adolescents' performance in different academic areas. Discussion centers on the impact of middle school reform efforts on student motivation and ways to ease the educational transitions. The chapter concludes by summarizing empirical work on the links between expectancies, values, and academic outcomes, self-regulation, and the decision to stay in school. (Contains 113 references.) (KB)