The classification of chronic childhood arthritis has challenged physicians for a century. Currently used classifications are all based on clinical characteristics and because they are frequently used imprecisely, communication of scientific data has been difficult. The new classification of the International League of Associations of Rheumatologists (ILAR) represents the results of an international effort to clarify the classification of this group of diseases based on identification of clinically homogeneous groups. This classification process is ongoing, and will reflect the results of scientifically based studies as they become available.