This chapter brings together evidence about the effect of primary health care on population mortality in low-, lower- and upper-middle- and high-income countries. Reviews of evidence completed by other researchers are summarised, experience in selected low-, lower- and upper-middle-income countries is described, and the findings are outlined of studies into the effect on population mortality of numbers of primary health care doctors per unit of population and broad measures collectively referred to as the strength of primary health care. The conclusion is that primary health care does affect population mortality, and the chapter goes on to introduce one mechanism by which this is brought about – the number of primary health care doctors available to serve the population.