Age-related change in processing speed has been linked directly to increases in reasoning as well as indirectly via increases in the capacity of working memory (WM). Most of the evidence linking change in speed to reasoning has come from cross-sectional research; in this article we present the findings from a 2½-year longitudinal study of 277 6- to-13-year-olds. On three occasions, speed of information processing was assessed with Visual Matching and Cross Out; WM was assessed with reading, listening, backward digit, alphabet, and operation span tasks; and nonverbal reasoning was assessed with Raven's progressive matrices. The results provided consistent evidence of direct links from processing speed to reasoning but inconsistent evidence for indirect links from speed to WM to reasoning. These findings suggest that variations in processing speed may constrain the development of reasoning, directly and perhaps indirectly. A video abstract of this article can be viewed at: https://youtu.be/nc0VlFdi468