Heat transfer deals with the rate of heat transfer between physical systems. Heat transfer processes set limits to the performance of all biological systems and their components. This chapter introduces three different types of heat transfer: conduction, convection, and radiation. In addition, microscale heat transfer, the transfer of thermal energy at the microscale level, is discussed. Furthermore, the transfer of thermal energy in biological systems such as human body is presented. Pennes bioheat equation is introduced and several case studies such as application of magnetic field in hyperthermia and application of ultrasonic wave in biosystems are presented.