The design and construction of structures in seismic areas requires knowledge of the seismic actions to which they can be subjected throughout their useful life. In general, it is not enough to establish what would be the maximum acceleration which could act upon the structure to be designed, but, in addition, the frequency with which the seismic phenomenon occurs and the damage it would produce should be known. In this monograph the probability of occurrence of earthquakes is studied regardless of the type of structure, of the population of the area, of the industrial level of the same, for the northeast area of the Iberian Peninsula; the results obtained are reflected in maps. The study is restricted to finding the probabilities of occurrence of earthquakes classified by their intensity (epicentral or felt) and, in spite of the approximation due to the use of the intensities, the results will be of great value for the making of design decisions. The methods for evaluating these probabilities of intensity occurrence are very varied, but basically there are two different conceptions of the methods of calculation: the deterministic methods and the probabilistic ones. The deterministic methods start from the recorded earthquakes and evaluate the effects of these using different coefficients. The probabilistic methods do not directly use the recorded earthquakes, but these data serve to establish certain laws of probability of occurrence that give rise to regional or zonal models of occurrence. The method presented herein has elements of both approaches, although always in the context of Bayesian statistics and is adapted to the special conditions of the northeast of the Iberian Peninsula or other areas where data are scarce and seismicity is moderate.