This article attempts to quantify the decline of Spain over the period 1500–1850. In contrast to earlier estimates that focus almost exclusively on Castilian agriculture, we look at trends in urbanisation and construct new measures of agricultural and aggregate output at both regional and national levels. A distinctive long-run behaviour is found across Spanish regions that rejects the identification between Castile and Spain. Per capita income grew in the sixteenth and early nineteenth centuries, while contraction and stagnation occurred in the seventeenth and eighteenth centuries. In the long run, output per head did not improve until the early nineteenth century. At the time of its imperial expansion Spain was a relatively affluent nation and, by 1590, was only behind the Low Countries and Italy in terms of per capita income. Spain's decline has its roots in the seventeenth century while its backwardness deepened in the first half of the nineteenth century.