The International Workshop on Wearable and Implantable Body Sensor Networks (BSN 2007) was held from 26th to 28th March, 2007 at the University Hospital Aachen, Germany. BSN 2007 is the fourth in the series of successful workshops launched three years ago at Imperial College in London. After the event was hosted by MIT in Boston last time, BSN 2007 returned to Europe and for the first time organized jointly with academia (RWTH Aachen University) and industry (Philips Research Europe, Aachen). The last decade has witnessed a rapid surge of interest in new sensing and monitoring devices for healthcare and the use of wearable, implantable and ambient devices for medical applications. The papers presented at BSN 2007 by leading scientists from computing, biotechnology, engineering and medicine address general issues related to on-body and in-body sensors. They discuss the latest technical developments and highlight novel applications of body-sensor networks in clinical settings, at home and on-the-move. Topics covered include new medical measurements, smart bio-sensing textiles, low-power wireless networking, system integration, medical signal processing, multi-sensor data fusion, and on-going standardization activities.