Musculoskeletal ultrasound has become a widely used imaging diagnostic tool both in the use of daily clinical practice and for clinical studies in monitoring treatment efficiency and predicting disease outcome. By US, detection of inflammatory soft tissue and erosive bone lesions is possible. Grey-scale and power Doppler ultrasound examination is more sensitive and more reliable than clinical examination. Furthermore, patients with unclear arthritic symptoms can be better diagnosed for arthritis by US than by clinical examination. This article gives an overview about the use of US in the diagnosis of early arthritis, especially early rheumatoid arthritis, its role as a prognostic assessor (structural damage), as a monitor for treatment response, as an detector of "real" remission, and a guide to injection procedure.