This book constitutes the refereed proceedings of the First European Workshop on Wireless Sensor Networks, EWSN 2004, held in Berlin, Germany in January 2004.The 24 revised full papers presented were carefully reviewed and selected from 76 submissions. Wireless sensor networks are a key technology for new ways of interaction between computers and the physical world around us. Compared to traditional networking, wireless sensor networks are faced with a rather unique mix of challenges: scalability, energy-efficiency, self-configuration, constrained computation and memory resources in individual nodes, data centricity, etc. This is one of a very small number of books entirely devoted to the presentation of cutting-edge R & D results in this exciting new area.