We cordially welcome you to the proceedings of the 2007 International S- posium on Ubiquitous Computing Systems (UCS) held at Akihabara, Tokyo, Japan. UCS has become a symposium for the dissemination of state-of-the-art research and engineering practices in ubiquitous computing with particular - phasis on systems and software. UCS 2007 was the fourth of this series of int- nationalsymposia.ThiswastheyearfortheNextGenerationNetwork(NGN)to be commercially launched so that the Internet could become the infrastructure forcommunicationsandcomputingsubstitutingtheNGNintelephonenetworks. The...