This is an investigation of the development of secondary students' beliefs about the nature of knowledge and learning, or epistemological beliefs, and the influence that these beliefs have on academic performance. An epistemological questionnaire that assesses students' beliefs about simple knowledge, certain knowledge, quick learning, and fixed ability to learn was modified and administered to more than 1,000 high school students. Factor analysis of students' responses replicated the factor structure found in previous research. Differences in epistemological beliefs among students across the high school years and between genders were examined. Belief in simple knowledge, certain knowledge, and quick learning decreased from freshman to senior year. Girls were less likely to believe in quick learning and fixed ability. Students' grade point average (GPA) was regressed on epistemological factors. The less students believed in quick learning, the higher the GPA they earned.