Musculoskeletal ultrasound has emerged as a key tool for the diagnosis, prognosis, and management of patients with RA (rheumatoid arthritis) and other rheumatic diseases. The most important sonographic findings in RA include erosions, effusions, synovitis, and tenosynovitis. Investigators have suggested various " optimal" numbers of joints to scan in RA to assess disease activity, gauge treatment response, provide prognostic information, and guide management decisions. The complexity of pediatric sonoanatomy has delayed its validation in juvenile idiopathic arthritis, yet ultrasound reliably measures the extent of synovitis/tenosynovitis and guides precise injections. © 2013 Elsevier Inc.