The book Why Are Some People Healthy and Others Not? The Determinants of Health of Populations represented a milestone in our evolving understanding of the determinants of population health. Building on Marc Lalonde's earlier A New Perspective on the Health of Canadians, it created a theoretical framework that could incorporate emerging evidence from a wide range of disciplines. Central to its authors' approach was the observation of heterogeneity, of the systematic differences in health observed when populations are partitioned on characteristics such as income, education, geographic region, etc. The universal observation of a social gradient, of a strong correlation between socio-economic class and health, led to a focus on how the social environment might influence health. Social position strongly influences both the stresses to which individuals are subject, and the resources available to cope with them. Furthermore, healthy and unhealthy responses to stress become "embedded", learned or conditioned both behaviourally and biologically, thus influencing health over the whole life course. The book's impact has been remarkable, not merely in academic citations but through its authors' subsequent work and strategic positions in Canadian health research organizations. The concept of "Population Health" has become part of our shared intellectual heritage.