Implantable Medical Devices (IMDs) are being embedded increasingly often in patients’ bodies to monitor and help treat medical conditions. To facilitate monitoring and control, IMDs are often equipped with wireless interfaces. While convenient, wireless connectivity raises the risk of malicious access to an IMD that can potentially infringe patients’ privacy and even endanger their lives. Thus, while ease of access to IMDs can be vital for timely medical intervention, too much ease is dangerous. Obvious approaches, such as passwords and certificates, are unworkable at large scale given the lack of central authorities and frequent emergencies in medical settings. Additionally, IMDs are heavily constrained in their power consumption and computational capabilities. Designing access-control mechanisms for IMDs that can meet the many constraints of real-world deployment is an important research challenge. In this paper, we review proposed approaches to the accesscontrol problem for IMDs, including the problem of secure pairing (and key distribution) between an IMD and another device, such as a programmer. (We also treat related technologies, such as bodyarea networks.) We describe some limitations of well-conceived proposals and reveal security weaknesses in two proposed cryptographic pairing schemes. Our intention is to stimulate yet more inventive and rigorous research in the intriguing and challenging areas of IMD security and medical-device security in general.