I have conducted research in the following places: the Knowledge & DB Lab (group, N.T.U. Athens), in GMD-IPSI (currently Frauhofer - IPSI), Germany, in Fern-Universitaet Hagen, in project VERSO(now GEMO) in INRIA/Paris, in IBM India Research Laboratory and in Max Planck Instistut fur Informatik (Saarbr cken, Germany). Currently I am supported by a Marie Curie Intra-European fellowship to work in area of "P2P Web Search", hosted by INRIA FUTURS (in GEMO group, headed by S. Abiteboul) in Orsay, Paris.