In this chapter a theory that describes the assets that help to protect health is presented. Assets, and their opposite – the conditions which create vulnerabilities to ill health – are located in the lifeworlds of ordinary human experience and the health benefits and disbenefits which accumulate over the life course. The lifeworld and lifecourse together are the bridge between social structure and individual human biology. Together they constitute the focal point where society and biology intersect and interact. The lifeworld and lifecourse are the mechanisms through which the social determinants of health produce biological outcomes in individuals. This is the vital link in the causal chain from the social to the biological and from society to individuals. Assets and vulnerabilities are the crucial mediating or intervening variables between the wider determinants of health and the human body and it is those intervening variables that produce individual differences in health.