This paper examines the evolution of the population density in Spain over the twentieth century. Using a homogeneous database of the population at a municipal level – elaborated from the eleven censuses carried out between 1900 and 2001 – the paper looks at the general characteristics of population concentration from various perspectives. Focusing on population density, we present empirical evidence supporting the hypothesis that, over time, Spain’s population has undergone progressive concentration, and that this process is still under way. Its main contribution is to offer quantitative support for phenomena which have already been well documented by specialists in more general terms.