In this chapter, we will discuss the two quantities that result from energy transfer across the boundary of a system: work and heat. This will lead into a presentation of the first law of thermodynamics. Work will be discussed in detail and will be calculated for several common situations. Heat transfer, often simply called “heat,” however, is a quantity that requires substantial analysis for its calculation. The study of heat transfer—conduction, convection, and radiation—is included at the appropriate places where it arises in our studies of thermodynamics and fluid mechanics.