In this paper, we describe the WellPhone, a smartphone with additional software, that is used as a personal health monitoring device. The WellPhone interfaces various health monitoring devices to the smartphone, and collects physiological data from those devices. It employs novel algorithms that perform statistical analyses, relate sequences of disparate measurements from different devices, and correlate physical activity with physiological measurements. The WellPhone provides feedback to the user by means of visualization and speech interaction, and alerts a caregiver, medical professional, or emergency responder, as needed.