The soybean cyst nematode (SCN; Heterodera glycines Ichinohe 1952) is one of the most economically important pathogens of soybean. The species impacts soybean production worldwide and causes significant yield loss even in the absence of visible aboveground symptoms. This diagnostic guide describes the host range, signs, symptoms, and geographic distribution of SCN, and includes information regarding pathogen identification, storage, and pathogenicity tests for this nematode.