Juvenile idiopathic arthritis (JIA) is the most common cause of chronic joint inflammation in childhood. The aetiology is unknown and the pathogenesis is multifactorial. JIA manifests itself in many various ways. It is a diagnosis of exclusion: other disorders need to be ruled out for a diagnosis to be made. MRI examination is playing an increasingly important role in making a correct early diagnosis and in assessing response to therapy. After 6 months JIA patients are classified, based on clinical characteristics and laboratory results, into one of the JIA categories according to the criteria of the International League of Associations for Rheumatology. Recent developments in therapy, such as starting biological treatment at an early stage, have led to an improvement in the prognosis of JIA and to structural joint damage occurring less often.