Although the etiology of Perthes disease remains unknown 100 years after its first description, there are many articles that describe the disease course, final outcome, and results of treatment. A system of classification of the extent and severity of the disease is essential to understanding variability of Perthes, and along with the age of the patient when first affected, is useful in predicting long-term outcomes. Published reports of treatment strategies and their success depend on effective classification of the disease severity and radiographic result at final follow-up concerning head sphericity, congruency with the acetabulum, and arthritis. This article reviews published articles that contain classification systems and details presently used systems that are helpful in understanding and in treating Perthes.