This book traces regional income inequality in Spain during the transition from a pre-industrial society to a modern economy, using the Spanish case to shed further light on the challenges that emerging economies are facing today. Regional inequality is currently one of the most pressing problems in the European Union, and this text presents a novel dataset covering 150 years to analyse long-run trends in regional per capita GDP. Spatial clustering and a new economic geography approach also contribute to the historical analysis provided, which points to the role played by spatial externalities and their growing relevance over time. To identify the presence of spatial dependence is crucial, not only for getting a better understanding of distribution dynamics, but also for economic policy purposes. What are the potential causes behind the disparities in regional per capita income and productivity? The authors answer this by comparing results with evidence available for other countries, chiefly France, Italy and Portugal, but is of global relevance.