“Of the Populousness of Ancient Nations” is by far the longest of Hume’s essays. Although it does not always receive the attention it merits, it is a very important text not only in relation to Hume’s political thought as a whole but also for a full understanding of the intellectual history of the long eighteenth century as it partook in a set of wide-ranging conversations about the causes of demographic growth in which T. R. Malthus, amongst others, became engaged. This chapter first revisits Montesquieu’s position on the issue of the relative populousness of ancient and modern nations to show not only the true nature of the Frenchman’s views and that of the dispute between him and Hume but also the extent to which Hume’s reading of Montesquieu provided the basis for the Scot’s reflections on republics, liberty, the status of women and slavery in that essay and elsewhere. It underscores the centrality of demography to political debates of the period.