The present discussion paper sets forward a model within the life course perspective of how the social becomes biological. The model is intended to provide a framework for thinking about such questions as how does social class get into the molecules, cells and tissues of the body to produce social class differences in life expectancy and cause of death? A categorisation of social exposures and biological processes is suggested; and some principles governing their inter-relations proposed. The paper ends by suggesting two public health applications of this approach.