Arthritis in childhood can be associated with low levels of physical activity and poor physical fitness. Children with arthritis may have decreased aerobic and anaerobic fitness, muscle weakness, low bone mass, and low bone strength. Suboptimal physical activity and exercise capacity may contribute to further deconditioning and disability, placing children with arthritis at risk for poor health outcomes. Recent studies suggest that exercise therapy is safe and does not worsen arthritis. Exercise therapy may improve function, quality of life, and physical fitness. However, little is known about the exercise prescription that is most effective to improve clinical outcomes in children with arthritis. This article reviews the current literature on physical activity, physical fitness, and exercise therapy in children with juvenile idiopathic arthritis.