In the treatment of chronic diseases, wireless Implantable Medical Devices (IMDs) are commonly used to communicate with an outside programmer (reader). Such communication raises serious security concerns, such as the ability for hackers to gain access to a patient's medical records. This brief provides an overview of such attacks and the new security challenges, defenses, design issues, modeling and performance evaluation in wireless IMDs. While studying the vulnerabilities of IMDs and corresponding security defenses, the reader will also learn the methodologies and tools for designing security schemes, modeling, security analysis, and performance evaluation, thus keeping pace with quickly-evolving wireless security research.