Two of the most interesting conceptual turns in Richard E. Snow's thinking called for: a broadening of the concept of aptitude to include not only cognitive processes, but also affective and cognative processes as essential for understanding academic performance and learning; and an exploration of the possibility that individual differences in learning and achievement emerge from dynamic person-situation transactions that unfold over time. The articles in this special issue address these "big ideas" through the lens of a study of high school students' achievement in science.