We propose a prospective observational study with two aims to determine the diagnostic value of musculoskeletal (MSK) ultrasound (US) in Juvenile Idiopathic Arthritis. Aim 1 will distinguish features of MSK US in JIA patients from normal findings in healthy individuals. Aim 2 will discern the features of active disease from inactive disease by following newly diagnosed JIA patients prospectively over a 1-year period with serial MSK US and clinical assessments.