Integrated Research in Grid Computing presents a selection of the best papers presented at the CoreGRID Integration Workshop (CGIW2005), which took place on November 28-30, 2005 in Pisa, Italy. The aim of CoreGRID is to strengthen and advance scientific and technological excellence in the area of Grid and Peer-to-Peer technologies in order to overcome the current fragmentation and duplication of effort in this area. To achieve this objective, the workshop brought together a critical mass of well-established researchers (including 145 permanent researchers and 171 PhD students) from a number of institutions which have all constructed an ambitious joint program of activities. Priority in the workshop was given to work conducted in collaboration between partners from different research institutions and to promising research proposals that could foster such collaboration in the future.