The evolution of wireless communications technologies such as Wi-Fi (IEEE 802.11) and cellular (LTE, 5G NR, etc.) has been largely driven by the need for greater data throughput. Increases in achievable data throughput are based primarily on either increased bandwidth and/or on higher modulation orders. Higher order modulation, in which more bits are transmitted with each symbol, require higher modulation accuracy, and therefore quantifying modulation accuracy has become one of the most important tasks in the design, test and debugging of modern radio frequency communications technologies. This paper provides an introduction to error vector magnitude (EVM), which is the primary metric of modulation accuracy.