The most important practical aspect of regional seismicity is its potential for structural damage. Although Fennoscandia is characterized by a fairly low rate of spatially scattered seismicity, the seismic hazard must be considered for sensitive structures like dams, mines, nuclear power plants, underground depositories for radioactive waste, oil platforms, etc. The main objective of the present study is to provide a detailed map of the median seismic hazard (horizontal PGA) for Fennoscandia for a mean return period of 475 years using modern computational algorithms. There are competing hypotheses as to the cause of earthquakes in Fennoscandia: release of stresses built up and propagated from the North Atlantic Ridge versus stress adjustment connected to the postglacial land uplift ( e.g. , see Wahlstrom, 1993). Occasional significant earthquakes have occurred, notably in 1759 in Kattegat between Denmark and Sweden, Ms = 5.6, and in 1819, 1866, and 1904 in Norway, Ms = 5.8, 5.7, and 5.4, respectively (magnitudes from NFR/NORSAR and NGI, 1998). Typical for an intraplate region, fault mapping and understanding of the tectonics of seismicity patterns are incomplete. Any regionalization of the investigated region, Fennoscandia, based on geological-structural elements and/or seismicity is therefore uncertain. In this study, a combination of different source regionalization models and region-independent models are used to determine the regional seismic hazard. Wahlstrom and Grunthal (2000) used a technique similar to that of the present paper to assign seismic hazard to Sweden, Finland, and Denmark. They also reviewed previous studies of seismic hazard in Fennoscandia. Improvements and changes incorporated in the present study include the addition of Norway, a stronger influence of nonregionalization models, and a statistically more prudent calculation of maximum expected magnitudes. Horizontal PGA values are calculated for a grid of points covering Norway, Sweden, Finland, and Denmark along with adjacent water areas. The …