In recent years, wearable and implantable medical devices (IMDs) have proven to be extremely effective in a variety of healthcare monitoring and therapeutic applications. In body-centric communications, various sensors, or medical devices take part, some of them are implanted into the human body, and others can be on-body in the form of wearable devices. In order to make the communication successful in the biological environment, antennas of the wearable and IMDs should be able to establish robust communication in the lossy biological environment to the outside-body devices. In comparison to conventional antennas, the main requirements of wearable and implantable antennas for robust communications are wide impedance bandwidth, multiband operation, optimum gain, and low specific absorption rate (SAR). Due to the electromagnetic coupling between the antenna and the biological tissues of the human body, it is quite challenging to achieve stable performance from wearable and implantable antennas. Metamaterials are artificial materials that are not found in nature and can provide extraordinary EM properties such as surface wave reduction, multiband behavior, compact size, gain enhancement, and wave polarization. In this chapter, different metamaterial-based implantable and wearable antennas are discussed. It has also been investigated how different metamaterials can improve antenna performance by improving bandwidth, multiband behavior, gain, and SAR inside the body or for on-body (wearable) applications.