Populations in the past behaved in diverse ways in terms of fertility, mortality and migration – the key elements of demographic dynamics. There are many variables which influence these dynamics, including environment and epidemiology, economic activity, urbanisation, reproductive decision making and war. These variables are socio-economically and culturally specific, and are therefore likely to impact differently on populations across time and place. Population historians of most periods in European history have long acknowledged such specificity and diversity in population dynamics and behaviour; in fact, established models that suggest ‘regional’ patterns of demographic behaviour and fail to take diversity into account have recently been challenged with data from a range of populations. Similarly, the notion that all pre-modern populations can be grouped together and be seen to behave in the same way as one another is no longer tenable. Accordingly, ancient historians must view the populations of the Graeco-Roman world against the backdrop of their environmental, socio-economic and cultural diversity. The populations of the areas discussed in this volume – Athens, Rome, the metropolises and villages of Ptolemaic and Roman Egypt, and the rural and coastal demes in Attica – all existed within specific contexts determining, at least in part, the variables shaping their population dynamics. For this reason we cannot categorise the range of populations of the Graeco-Roman world along with all pre-modern European populations, nor can we see them as making up one distinct and homogeneous category of their own. On the other hand, there are lots of commonalities between the populations of the ancient, pre-modern and more recent past. The value of comparative research on population lies in establishing what these common relationships are, and the ways in which an understanding of one population in its particular context can help to develop a fuller picture of another. For the ancient historian, for instance, a study on population in early modern England can suggest not only the differences but also the similarities in the ways in which individuals, families and populations influence and respond to social and economic change.