Boxwood blight, caused by Calonectria henricotiae and C. pseudonaviculata, is an extremely destructive disease of cultivated and native Buxus species (boxwood) worldwide. The disease is widespread in North America, Europe, and Southwest Asia. Early and accurate diagnosis of boxwood blight is the most effective tool used to prevent spread of the disease, and it is the first step for development and implementation of appropriate control measures. This diagnostic guide provides up-to-date information on how to diagnose and identify boxwood blight and also offers an overview of methods used to isolate the pathogen, store cultures, and evaluate pathogenicity of the causal agents.