The classification of the juvenile arthritides is an evolving process which has yet to achieve its ultimate goal of delineating biologically distinct disease groups with predictable outcomes and responses to treatment. There have been considerable advances, however, since the first attempts at classification over 100 years ago, as the clinical heterogeneity of juvenile arthritis has become understood. The aim of this chapter is to highlight the historical milestones in classification, the most recent proposals for developing a unified, international classification, and the published literature aimed at evaluating the classification process. In conclusion, it is recommended that all clinicians involved in paediatric rheumatology take part in the ongoing process of juvenile arthritis classification by the prospective collection of standardized clinical data.