Heat transfer plays a significant role in various physical systems, spanning from mechanical and electronic devices to geothermal energy in subsurface environment. In studying heat flow in a physical system, the amount of heat transferred to and from the system due to thermal interaction with other systems or with the environment is a quantity that is essential for engineering design and analysis. A quantitative description of the involved thermal processes requires knowledge of the concept of thermal equilibrium, Thermalenergy" \r "TEBK5900thermal energy, and heat transfer. The laws of thermodynamics provide relationships between temperature, heat transfer, and energy, and thus it is vital that these laws are used as the basis for formulating mathematical models to describe heat flow in physical systems. This chapter highlights the physics of heat transfer, energy balance, and some relevant initial and boundary conditions.