This political history studies the phenomenal growth of the modern British state's interest in collecting, collating and deploying population data. It dates this biopolitical data turn in British politics to the arrival of the Labour government in 1964. It analyses government's increased desire to know the population, the impact this has had on British political culture and the institutions and systems introduced or modified to achieve this. It probes the political struggles around these initiatives to show that despite setbacks along the way and regardless of party, all British governments since the mid-1960s have accepted that data is the key to modern politics and have pursued it relentlessly.