- Phase 1. A pediatric rheumatologist will examine 25 subjects with a spectrum of arthritis severity after informed consent is obtained. Large and medium joints, such as knees, ankles, wrists, and elbows, will be assessed for active arthritis, as defined by swelling or limitation of motion with pain or warmth. Grading of clinical severity is derived from the ACR Pediatric 30 and will assess for swelling on a scale of 0-3 and limitation of motion with warmth or pain on motion on a scale of 0-3. Based on this grading, a Clinical Severity Score for inactive, mild, moderate, or severe arthritis will be obtained for each joint. Following the clinical evaluation, a pediatric rheumatologist who has received training in musculoskeletal US (PI) will image the joint. A portable US machine with a high frequency probe (6-18 MHz) and power Doppler capability will be used to image the target joints and to identify anatomical signs suggestive of arthritis. At least 2 views, coronal and transverse, of each joint will be obtained and recorded per musculoskeletal US specifications as described by the European League Against Rheumatism (EULAR). US images will be saved under a subject code on the US machine hard drive. These images will be reviewed with a consulting rheumatologist with expertise in musculoskeletal US (J. Nishio, MD). US determination of synovitis (synovial thickness with Doppler enhancement), effusion, erosions, and tenosynovitis will be based on definitions established by the Outcome Measures in Rheumatoid Arthritis Clinical Trials (OMERACT) 7 consensus group for adults with rheumatologic disease. An US score for arthritis will then be developed based on the gold standard of clinical exam. The US score will assess for active arthritis as defined by synovitis on a scale of 0-3 and/or effusion on a scale of 0-3. In addition, other significant abnormalities in the joints will be evaluated, including absence or presence of erosions and tenosynovitis. The result of this phase of Aim 1 will be the creation of an US Severity Score for inactive, mild, moderate, or severe arthritis in a single large or medium joint that is consistent with the Clinical Severity Score.