The relevant properties of the human body from the very high frequency (VHF) to the X band are studied and dominant propagation modes around the body are discussed in this chapter. Based on these data, approximate human torso models are built for each propagation mode using multiple concentric cylinders emulating the skin, muscle, organ, and the like. After this study, a simpler two-layer model of the human body is proposed for efficient calculations without loss of accuracy and used in the analysis of the epidermal sensor. The radio frequency (RF) properties of human body tissues affect propagation, reflection, attenuation, and other behaviors of electromagnetic fields around the body. Body-centric wireless communications can be divided into three types based on the interaction between the body and the EM field. In each case, different propagation modes are dominant.