The authors report on two longitudinal studies that explored changes in aspirations, experiences, and attitudes in science over three years among students in California public middle and high schools. One study surveyed an ethnically and economically diverse group of over 400 students who participated voluntarily in 7th through 9th grades, and then interviewed 48 students. The study identified four trajectories that students' interest in science, engineering, and medical (S/E/M) careers took over time and described factors that affected their aspirations. The other study looked in depth at what happened to an equally diverse group of 33 10th graders who were very interested in a S/E/M career. It followed them through the 12th grade to discover what happened to their aspirations in high school.