The size, growth, density, distribution, and age structure or composition of a population is broadly dependent on three drivers: fertility (numbers of births), mortality (number of deaths), and migration (number of moves in and out of that population). ‘The drivers’ explores the ideas behind our understanding of these drivers on which most demographic analysis is focused. It explains that the main measures of fertility in a population are the Crude Birth Rate and the Total Fertility Rate. The main measures of mortality are the Crude Death Rate and life expectancy. Migration is different in that while mortality and fertility are typically studied within a closed population, migration introduces the notion of an open population.