This volume contains the proceedings of EWSN 2007, the fourth European c- ference on Wireless Sensor Networks. The conference took place at TU Delft, January 29–31, 2007. Its objective was to provide a forum where researchers with di?erent experience and background,from hardware to applications, would presentanddiscussthelatestdevelopmentsintheexciting?eldofwirelesssensor networks. Since the interest in sensor networks has been rapidly expanding, it was no surprise that EWSN received a record number of 164 submissions, of which 22paperswereselectedforthe?nalconference.Itwasapleasuretoobservethat, although based in Europe, the conference serves as a truly international forum with submissions originating from all ?ve continents: 35% from Europe, 35% from Asia, 26% from America, 3% from Australia, and 1% from Africa. The - lectionprocessinvolvedmorethan500reviewswithmostpapersbeingevaluated by at least three reviewers. The ?nal program covered a wide range of topics, grouped into seven sessions: networking, tracking, algorithms, applications and support, medium access control, OS and tools, and localization. In addition to the papers published in these proceedings, the conference also included a poster and demonstration session, of which separate proceedings are available. Tutorials and keynotes complemented the program, together making for a truly interesting conference. In closing, we would like to express our sincere gratitude to everyone who contributed to EWSN 2007. In particular, the members of the Program C- mittee and external reviewers responsible for the strong technical program, the local TU Delft people for streamlining the conference logistics, and Springer for their excellent cooperation in putting these proceedings together.