This monograph describes a conceptual framework necessary for the formulation of seismic risk and of vulnerability studies. The Italian methodology of the vulnerability index is described for unreinforced masonry buildings and is applied to an area of the downtown of the city of Barcelona, Spain. A Geographical Information System (GIS) is used which allows automating the process completely and facilitates obtaining the results. Studies of observed seismic vulnerability, based on the damage caused by earthquakes in the past, are performed and their relationships with the previous method is examined. An efficient scheme is proposed to carry out post-earthquake investigations which is applied to the case of the two earthquakes that occurred in the region of Almeria in December 1993 and January 1994; it is explained how to make effective use of the results. To achieve this objective, data on the effects of the mentioned earthquakes has been collected and processed with the objective of obtaining the first observed vulnerability function for Spain. This function would allow the evaluation of future losses due to earthquakes in Spain. In this way, both a seismic risk study methodology and a complete procedure to evaluate the seismic vulnerability of an entire area in an urban will be available.