Purpose: This article draws from cumulative disadvantage and life course theories to develop a new theory for the social scientific study of aging. Design and Methods: Five axioms of "cumulative inequality (CI) theory" are articulated to identify how life course trajectories are influenced by early and accumulated inequalities but can be modified by available resources, perceived trajectories, and human agency. Results: Although the concept of CI has attracted considerable attention among social scientists, it holds promise for integrating additional disciplinary approaches to the study of aging including, but not limited to, biology, epidemiology, and immunology. The applicability of CI theory to gerontology is illustrated in research on the early origins of adult health. Implications: Primary contributions of the theory to gerontology include greater attention to family lineage as a source of inequality; genes, gestation, and childhood as critical to early and enduring inequalities; the onset, duration, and magnitude of exposures to risk and opportunity; and constraints on generalizations arising from cohort-centric studies.