This chapter explores both theoretical debates about states and populations in the classical tradition of political philosophy and the actual practices of ancient polities in relation to their human resources, arguing that Michel Foucault’s influential idea of the ‘invention of population’ in the eighteenth century needs serious revision. Not only were questions of how large the population should be, its optimal character and constitution, fundamental to the utopian visions of Plato and Aristotle, for example, but both had their ideal cities regulating marriage, child-bearing and child-rearing in various ways. Ancient states were also concerned with matters of population in practice. The Roman Republican census enumerated and ordered the citizen-body, and provided a key opportunity to encourage marriage and child-bearing, for instance; and classical Sparta also incentivised large citizen families, within an explicitly eugenic frame, in which the state organized to optimize its offspring, through both positive measures, such as those aimed at ensuring mothers were physically robust, and the negative mechanism of exposing weak and sickly infants. All this indicates that ancient engagements with actual and notional populations was much more rich and complex than Foucault allows, something worth studying in its own right.