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 this special issue. The articles 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. Each paper picks up one of the themes identified--trait complexes, ATIs, process analyses, and spatial ability--and has a strong quantitative, empirical foundation but is nested within appropriately complex theoretical frameworks.