The lifelong manifold process of aging implicates biological, psychological, social, and environmental factors that interact over time and across place in complex ways to direct and temporally organize the shapes and boundaries of lives. As such, aging is a long, broad, and deep process: long, because it occurs continuously across the life span; broad, because it continuously integrates diverse factors from across levels of observation; deep, because it is never fully and directly observable as an ongoing generative process. Over the last two decades, theory building in aging inequality has focused on defining the role of health in the aging process. Arguably, health is now the core metric of aging; the diverse and complex patterns of disease, disability, and mortality with age have become the central problem for aging researchers, especially those concerned with social inequality and its pervasive and enduring effects.