Juvenile idiopathic arthritis (JIA) is the most common rheumatic disease in children. Studies of the immunopathogenesis in JIA has mainly focused on the adaptive immune system, while less is known of the role of the monocytes. JIA is considered a chronic disease, although only about 50% of participants in long-term follow-up studies have active disease as adults. The treatment arsenal for JIA has expanded during the last three decades, but if this has improved the long-term outcome is not known.When a diagnosis of JIA is confirmed almost all children and parents ask the same important questions: “Why did I/my child get this disease?”, “What will happen to me now?” and “Will I ever get well?”. The overall aim of this thesis has been to study different aspects of these questions – from the pathophysiological role of monocytes and their function in blood and the synovial environment, to incidence, outcome measures with short- and long-term perspectives, risk of depression and anxiety, and mortality.Children with active oligoarticular JIA display monocytes of a mixed pro- and anti-inflammatory polarization pattern in the synovial environment, with reduced capacity to phagocytize, unique for this JIA subtype. This supports the hypothesis that oligoarticular JIA should not be considered as a pediatric version of adult rheumatic arthritides.The mean annual incidence rate in Skåne (the southernmost region of Sweden) 1980 – 2010 was 9.9/100,000 children