Background/Context: Given the importance of increasing student participation in science, technology, engineering, and math (STEM), there is a need to understand how factors such as student's attitudes toward math and science in middle and high school are linked to their later college and career choices. Purpose/Objective/Research Question/Focus of Study: The purpose of this study is to describe heterogeneity in math and science attitudes and how these attitudes change through middle and high school. This study contributes to the literature by using measures from grade levels where student attitudes might be particularly vulnerable to shifting; examining the extent to which attitudes are persistent for particular groups of students; comparing the stability of attitudes across multiple domains (mathematics and science), which are intertwined in the pursuit of STEM careers; and relating attitudes to STEM career attainment, an outcome that occurred some 20 years later. Research Design: We used the Longitudinal Study of American Youth (LSAY) to examine the development of student attitudes in middle and high school and the relationship of those patterns to academic achievement and career choices. We used latent class analysis and latent transition analysis to study change over time. Findings/Results: Based on latent class analyses conducted for seventh, 10th, and 12th grade independently, four attitudinal profiles were identified at each grade level that varied in their affinity toward mathematics and science: very positive, qualified positive, neutral, less positive. The stability of these four attitudinal profiles at each grade level varied over time. Using latent transition analysis, approximately 40% of students in this study did not change their attitudes from seventh through 12th grade. Of those who changed their attitudes, change occurred prior to 10th grade and changed toward more negative attitudes. This pattern of change varied depending on ethnicity and gender. Conclusions/Recommendations: Findings suggest that although a proportion of students have consistently positive or consistently negative attitudes toward mathematics and science, there is a larger group of students with malleable attitudes. These findings emphasize the importance of encouraging and supporting early positive attitudes toward mathematics and science, particularly for ethnically underrepresented females because of the impact on outcomes such as achievement and STEM career attainment.