Modern techniques for treating patients are very dependent on medical devices.  Medical devices include sensors, which provide vital information about patient state; actuators that effect treatment; and decision support systems that help clinical personnel in planning the treatment.  Increasingly, devices used in treating a patient are interconnected, forming a patient-centric cyber-physical system (CPS).  Medical CPS hold out the promise of improved patient care with fewer opportunities for human error and decreased treatment costs.  In particular, medical CPS enable the development of physiologically closed-loop approaches that automatically deploy safety measures in response to changes in the patient's condition.  At the same time, deployment of such medical CPS presents a variety of challenges.  This lecture will discuss design, deployment, and regulatory challenges and suggest possible ways to overcome then, motivated by several on-going projects at the University of Pennsylvania.  We will start with safety considerations for individual devices, such as infusion pumps and pacemakers; explore the implications of interoperability in collections of medical devices used in a treatment of a patient; and finally consider some of the clinical applications that are enabled by interconnecting medical devices.