Wirelessly networked systems of intra-body sensors and actuators could enable revolutionary applications at the intersection between biomedical science， networking， and control with a strong potential to advance medical treatment of major diseases of our times. Yet， most research to date has focused on communications along the body surface among devices interconnected through traditional electromagnetic radio-frequency （RF） carrier waves; while the underlying root challenge of enabling networked intra-body miniaturized sensors and actuators that communicate through body tissues is substantially unaddressed. The main obstacle to enabling this vision of networked implantable devices is posed by the physical nature of propagation in the human body. The human body is composed primarily （65 percent） of water， a medium through which RF electromagnetic waves do not easily propagate， even at relatively low frequencies. Therefore， in this article we take a different perspective and propose to investigate and study the use of ultrasonic waves to wirelessly internetwork intra-body devices. We discuss the fundamentals of ultrasonic propagation in tissues， and explore important tradeoffs， including the choice of a transmission frequency， transmission power， and transducer size. Then， we discuss future research challenges for ultrasonic networking of intra-body devices at the physical， medium access and network layers of the protocol stack