Internal energy is associated with the microscopic and collective random motion of particles constituting the media. Energy of the collective random motion of particles is heat. Heat flows across the boundary of a system. The net energy transfer by heat, work, and mass is equal to the sum of the change in internal, kinetic, potential energies. Enthalpy is used in many fluid flow systems. Only changes in internal energy and enthalpy can be calculated. Measurement of heat change by using calorimeter is briefly discussed. Estimations of energy and temperature changes of air using the psychometric chart are briefly discussed. Heat transfer and the concept of exergy in fluid flow systems are discussed briefly.