Radio frequency identification (RFID) technology is rapidly emerging within bio-medical engineering applications. Particularly in harsh and challenging environments, RFID provides fast and reliable item identification and tracking solution. In a typical passive RFID system, objects to be identified are marked with tags, consisting of an antenna and an integrated circuit. In the future, wearable RFID tags integrated with daily clothing could provide multi-functional garments for real-time remote bio-monitoring of human vital signs without any human involvement. At ultra-high frequencies (UHF), the human body is electrically a challenging application environment for the passive tag due to its high permittivity and high losses. The human body in close proximity tends to interact with the tag in a disadvantageous manner, resulting in severe overall tag performance degradation. This book presents a novel approach for the development of a UHF human arm model for wearable antenna applications. It is verified that the human arm model can be used as a powerful tool for future wearable antenna design and optimisation.