Juvenile idiopathic arthritis (JIA) is not a disease but an exclusion diagnosis that includes all forms of chronic arthritis of unknown origin with onset before 16 years of age. The current classification identifies several different categories. While some of them appear to represent rather homogeneous entities others seem still to include heterogeneous conditions. The advent of the new biological treatments has dramatically changed both the observed responses to treatment and the expectations of treatments. International research networks of paediatric rheumatology have contributed to fostering the conduct of controlled clinical trials and also the development of validated outcome measures. However, despite a dramatic advance in the understanding of JIA categories, pathobiology and treatments, much remains to be done.