In social epidemiology, to speak of theory is simultaneously to speak of society and biology. It is, I will argue, to speak of embodiment. At issue is how we literally incorporate, biologically, the world around us, a world in which we simultaneously are but one biological species among many—and one whose labour and ideas literally have transformed the face of this earth. To conceptualize and elucidate the myriad social and biological processes resulting in embodiment and its manifestation in populations' epidemiological profiles, we need theory. This is because theory helps us structure our ideas, so as to explain causal connections between specified phenomena within and across specified domains by using interrelated sets of ideas whose plausibility can be tested by human action and thought.1–3 Grappling with notions of causation, in turn, raises not only complex philosophical issues but also, in the case of social epidemiology, issues of accountability and agency: simply invoking abstract notions of ‘society’ and disembodied ‘genes’ will not suffice. Instead, the central question becomes: who and what is responsible for population patterns of health, disease, and well-being, as manifested in present, past and changing social inequalities in health?