The current work models urban growth in the continuous built-up areas of 47 Spanish cities from the mid-nineteenth century through to the present day. We did this by compiling a comprehensive Geographic Information System (GIS) dataset, based on a series of historic maps and aerial images, and then used this to study urban growth and to make spatial comparisons. Our chosen indicator of expansion: population density, was calculated by dividing the total population of each city (based on its municipal area) by its built-up area during each period. Our results revealed four different stages of growth, each of which was characterised by a certain political and economic reality. They showed the clogging up of the walled city, the shaping of the urban ensanches, the maturity of the compact city and the process of metropolisation.