Starting with the principle of rocket propulsion, this chapter is devoted to the mathematical derivation of the ideal rocket equation and defining rocket engine performance parameters. The second half of the chapter reviews basic concepts of thermodynamics and fluid mechanics to introduce the main equations that define the rocket as a thermal engine, including the performance parameters such as thrust, specific impulse, characteristic velocity, and thrust coefficient in terms of nozzle expansion parameters. Sample problems illustrate the main ideas that comprise the conceptual design of rocket engines.