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 maturity of the Internet encourages the research and development of the nextcomputing systems,where ubiquitous computing is recognizedasone of the most promising computing paradigms. This symposium was organized by IPSJ SIGUBI, IEICE USN and UCN, Korea, in cooperation with the IEEE Tokyo Section, IPSJ SIGEMB, IEICE Smart Info-media Systems Technical Group, and Human Interface Society. It was also sponsored by Ubiquitous Networking Forum, Nokia, NTT, SCAT, IISF and TAF. Thisyear,wehad96submissionsfrom18countries.TheProgramCommittee reviewed all the papers carefully and then selected 16 full papers and 8 short papers. The very low acceptance rate of about 22.6% clearly demonstrates the high quality of the conference, and this tradition will continue in the upcoming conferences. Two distinguished speakers were also invited for keynote speeches, who enlightened the audience on ubiquitous computing and applications. The high-quality technical program of UCS 2007 depends very much on the precise and stringent review process. The Technical Program Committee c- sistedof62excellentmembers.Mostreviewswarealmostofjournalpaperreview quality, and the paper selection was very serious and strict. Along with the symposium, we also offered a workshop and a poster session.