The legacy of Matilda White Riley to the study of aging encompasses multiple disciplines and extends to multiple domains within these disciplines. Although her greatest intellectual legacy is in sociology, she presented a compelling vision of the need for other disciplines to consider the role of social forces in shaping both aging as an individual, lifelong process and age as a feature of culture and social systems. This article reviews Riley's theoretical contributions in four areas: (1) articulating age and social systems; (2) identifying fallacies in the interpretation of research on aging; (3) theorizing about social change and structural lag; and (4) presenting social possibilities related to age integration. We conclude by considering briefly the reach of her legacy beyond sociology-in collaborating across disciplinary boundaries, in encouraging the development of sound longitudinal data archives, and in developing an institutional infrastructure to support and sustain research on aging in the social and behavioral sciences. Although short of physical stature, Riley's contributions to gerontology are enormous. Gerontologists from many disciplinary backgrounds have been informed by and rely upon these insights and thus share the advantage of "standing on the shoulders of a giant."