This chapter covers a broad range of topics within Wireless Body Area Network (WBAN) digital twins, encompassing modeling and simulation of tissue/organ dielectric properties, specific absorption rates, signal path losses, and heat generation due to signal propagation in the human body. This kind of digital twin (DT) was very important in the development of the Internet of Things (IoT) in the field of smart health, such as wireless wearable sensors and wireless implant devices. By DT, the appropriate wearable sensor position and its performance can quickly be identified. On the other hand, the power level of the propagated wireless signal can be initially estimated to optimize the efficiency of the implant devices and the safety of the body tissues/organs. In addition, this chapter proposes a graphical user interface-based simulation framework, which includes dielectric human tissue/organ models and path loss models, for realizing a digital twin ecosystem.