Unemployment has had a strong impact in western societies in the last twenty years. The high levels reached during the 1980s (a period in which Spain had the highest OECD records) made it a primary concern in public polls and one of the main objectives of social and economic policy. We can count today in millions the publications and reports that analyse unemployment, comparing it across countries, regions and localities. Investigations that examine the history of unemployment also number in hundreds. This literature is the fruit of varied research carried out across different disciplines; from economists who have tried to explain its evolution on the basis of different variables, to social historians who have presented it as a direct cause o f social mobilisation. When w e speak of unemployment it is assumed that we refer to a very clear thing. Unemployment has become a concept o f collective reference the meaning of which does not seem to require further explanation. But if we take a closer look, we can soon detect the multiple dimensions that such a concept has acquired over time. For although the term unemployment emerged in western vocabularies around the end of the nineteenth century in order to describe involuntary lack of work, the concrete identification of the unemployed has undergone important variations in different historical and political contexts. The inherent polysemy of the concept of unemployment and the heterogeneity of its referents poses a problem for many researchers who try to chart its historical evolution. However, this very same variety has been used by a series of recent investigations that attempt precisely to give account of the history of statistical categories and to relate these to a broader socio-political context. This thesis is inserted within that framework. It deals with the history of a statistical category, paro [unemployment] elaborated through the categorisation of the parados [unemployed] in Spain. The approach undertaken is deeply historicist and based on the proposals o f socio-histoire.