Richard Snow's research influenced many students and colleagues, both directly through his findings and indirectly by inspiring others to carry on the work. A cross-section of his influence is represented in the articles presented in this special issue. Snow was guided by a respect for empirical, quantitative approaches, applications, and a concern for large issues. We present several themes in his work, including the importance of multivariate considerations of individual differences, adapting instruction to individual learners, a process understanding of aptitude, and an enlarged role for spatial ability.