Promoting an understanding of multiplication of fractions has proved difficult for mathematics educators. I discuss a research study aimed at developing a concept of multiplication that supports both multiplication of whole numbers and multiplication of fractions. The study demonstrates how domain-specific theories grounded in two major psychological theories contribute to the development of an empirically based approach to developing this concept. Specifically, the researchers used Learning Through Activity, grounded in constructivism, and aspects of the Elkonin-Davydov Curriculum, grounded in Russian activity theory (sociocultural theory). [For the complete volume, "Constructing Number: Merging Perspectives from Psychology and Mathematics Education. Research in Mathematics Education," see ED616587.]