In this paper we present the technical requirements and system issues for wireless medical body sensor networks (BSNs). Design guidelines are driven by the need to improve ambulatory patient monitoring and care while reducing logistic constraints for patients as well as healthcare professionals. We present our study on three key components of medical BSN: On-body wireless link (to characterize the RF channel for body worn wireless devices), Coupling between bodies (to characterize the RF interaction between bodies) and coexistence of Medical BSNs in the RF spectrum. Results and conclusions are presented through simulation and measurement studies. We also discuss our FCC petition for spectrum allocation.