An unexpected brittle failure of connections and of members occurred during the last earthquakes of Northridge and Kobe. For this reason a heightened awareness developed in the international scientific community, particularly in the earthquake prone countries of the Mediterranean and Eastern Europe, of the urgent need to investigate this topic. The contents of this volume result from a European project dealing with the 'Reliability of moment resistant connections of steel frames in seismic areas' (RECOS), developed between 1997 and 1999 within the INCO-Copernicus joint research projects of the 4th Framework Program. The 30 month project focused on five key areas: *Analysis and syntheses of research results, including code provisos, in relation with the evidence of the Northridge and Kobe earthquakes; *Identification and evaluation through experimental means of the structural performance of beam-to-column connections under cyclic loading; *Setting up of sophisticated models for interpreting the connection response; *Numerical study on the connection influence on the seismic response of steel buildings; *Assessment of new criteria for selecting the behaviour factor for different structural schemes and definition of the corresponding range of validity in relation to the connection typologies. The INCO-Copernicus Project brought together Europe's 35 leading structural, architectural and material engineers to develop the basis for future design of moment resistant connections in seismic areas. This book, the published results of the project, is a must have for engineers who want to be at the cutting edge of design in seismic areas.