Both national and regional GDP are relatively recent economic indicators: they were systematically produced by statistical offices only well after the end of the Second World War. For earlier periods, GDP at any geographical level can be reconstructed only by using data that were not gathered for this purpose. Estimating historical regional GDP relies on even thinner evidence than estimating national GDP. This chapter provides an overview of the existing literature devoted to estimating historical regional GDP and discusses the methodological challenges faced by scholars. Finally, it illustrates a case study of the Swedish NUTS-3 regions, for which regional GDP estimates in selected benchmark years are available from 1571 to today. These estimates represent the longest series of regional GDP data ever compiled for any given country, giving a unique perspective on regional inequality from pre-industrial times to the present.