This paper examines a broad set of beliefs about achievement and the relationship of those beliefs to performance and future plans in mathematics and English. The two main purposes of the analysis are (1) to assess gender and grade-level differences in children's achievement beliefs as they proceed through junior and senior high school, and (2) to assess an expectancy-value model of children's achievement beliefs that specifies how those beliefs relate to children's course performance and course enrollment decisions, in order to more clearly identify the achievement beliefs that are critical mediators of performance and persistence in math and English. Children in grades 5 through 12 completed questionnaires assessing their achievement beliefs and attitudes in mathematics and English over a 2-year period. Results of the analysis showed that (1) boys had higher expectancies than girls for both current and future math courses, while girls had more positive beliefs about English; and (2) children's beliefs about math were less positive at each successive grade level, whereas their interest in English increased at higher grade levels. Many of the major tenets of the model received support. Children's expectancies for success and particularly their task value relate to their intentions to take more math, whereas ability perceptions are strongly related to both expectancies and values. Analyses of English beliefs showed similar results. Tables showing these results are included, along with a diagram of the expectancy-value model of student attitudes. (TE)