Mobile and wearable systems for monitoring health are becoming common. If such an m-Health system knows the identity of its wearer, the system can properly label and store data collected by the system. Existing recognition schemes for such mobile applications and pervasive devices are not particularly usable - they require active engagement with the person (e.g., the input of passwords), or they are too easy to fool (e.g., they depend on the presence of a device that is easily stolen or lost). The authors present a wearable sensor to passively recognize people. Their sensor uses the unique electrical properties of a person's body to recognize their identity.