This short course is intended to provide a straightforward explanation of how a reciprocating compressor works relying heavily upon the pressure-volume diagram as a point of reference. The discussion begins with a thorough review of pressure-time and pressure-volume diagrams. Next, how capacity is determined and calculated is discussed, which requires a thorough explanation of volumetric efficiency. How much energy is required to compress a certain volume of gas is then explained, which involves detailed discussions of adiabatic, valve loss and friction power - again relying heavily on the pressure-volume diagram. The short course then delves into discussions of compression efficiency, how changes in pressure and speed affect the pressure-volume diagram, how the mole weight of the gas being compressed affects the pressure-volume diagram, and discharge temperature, multi-stage compression, rod load and finally gas pulsation. The course should be an excellent introduction for the inexperienced and a solid refresher for the more experienced.