We develop a quantitative theory of mortality and population dynamics. We emphasize individuals' decisions to reduce their mortality by adopting better health technology. Adoption becomes cheaper as more individuals use better technology. It also confers a dynamic externality by increasing the future number of individuals who use the better technology. Our model generates a diffusion curve whose shape dictates the pace of mortality reduction. The model explains historical trends in mortality rates and life expectancies at various ages and population dynamics in Western Europe. Unlike Malthusian theories based solely on income, ours is consistent with the observed disconnect between mortality and income. Unlike Beckerian theories based solely on fertility, ours accounts for the observed acceleration in population.