Since 1801 the British government has counted the population once every ten years. Only the Second World War has interrupted this practice, making the census one of the most enduring administrative institutions of the modern British state. This dissertation is about why legislators and political economists first sought to quantify demographic change in the early nineteenth century. The first chapter explains the administrative organisation of census-taking under John Rickman, who directed the first four censuses. The second chapter examines the legislative origins of census-taking in eighteenth-century Britain. It compares the efforts of two backbenchers, Thomas Potter and Charles Abbot, to establish a national census in 1753 and 1800. The third chapter analyses the pre-census empirical basis of fiscal policy during the 1790s, paying patticular attention to William Pitt the Younger's use of political arithmetic to estimate the yield of Britain's first income tax. The fou1th chapter examines the function and limitations of the population data used by four national accountants - Benjamin Bell, Henry Beeke, J. J. Grellier and Patrick Colquhoun - in their responses to Pitt's new tax. The fifth chapter re-assesses the economic and social thought of Robet1 Southey, whose opposition to T. R. Malthus's Essay on the pr;ndple of populahon, and especially its commitment to poor law abolition, arose from a fundamental disagreement about the state's role in welfare provision. The sixth and seventh chapters consider the relationship between information gathering and state formation. Chapter six quantifies the number and range of printed accounts and papers produced by the House of Commons in the early nineteenth century. It challenges previous analyses which have used public expenditure and statute-making as measures of state formation. The final chapter explores how census data was used to determine the redistribution of parliamentary representation that took place as a result of the 1832 Reform Act. Employing a diverse range of methodologies and sources, this study contributes to histories of economic thought and state formation by revealing the extent to which political arithmetic converged with Smithian political economy during the French revolutionary and Napoleonic wars. This convergence proved sho1t-lived, however, and early nineteenthcentury political arithmetic was consigned to historical oblivion by the world 's first professional economist, John Ramsay McCulloch. Nonetheless, reasoning by 'number, weight, or measure', paiticularly in respect of population, challenged and transformed the conduct of parliamentary business in this period, leading to the legislative dissolution of the existing electoral system in 1832.