This chapter presents a snapshot of the regional output (GDP/GVA) structure of the British Isles using data for 14 census years between 1861 and 2011. It summarizes the steps employed to make the estimates of United Kingdom (UK) regional GDP. The chapter examines the structure of regional GDP in the UK and its development: a clear pattern emerges. The estimates require three sets of data: labour force by sector and region; aggregate GDP and aggregate GDP by sector; and productivity relatives usually obtained from wage relatives comparing sector wages in each region with the UK sector wage. From the 1990s, the fall in London’s share of GDP was decisively reversed as deregulation of financial markets allowed the city to become the world’s largest foreign exchange, banking and financial centre. Dublin’s GDP per head is clearly on a par with London, and both cities have much in common – financial centres, administrative centres, major tourist attractions and cosmopolitan populations.