Rheumatoid arthritis is a chronic inflammatory arthritis with the incidence reaching up to 1% of the population. The treatment of rheumatoid arthritis (RA) has improved dramatically over the last years with the introduction of TNF (tumor necrosis factor) alpha inhibitors. Joint damage and functional impairment are highly important adverse outcomes of rheumatoid arthritis. They have been repeatedly shown to be associated with clinical disease activity, in particular with swollen joint counts and acute-phase reactant levels, as well as composite measures of disease activity in which these variables are included as components. OMERACT outcome measures in rheumatology group published consensus US definitions for common pathologic lesions observed in patients affected by RA, each of them is discussed, effusion, synovitis, bone erosion, hypervascularity and tenosynovitis. The use of musculoskeletal ultrasound (MSK-US)is growing more and more between radiologists and rheumatologists for the early diagnosis and therapeutic follow up of rheumatoid arthritis and US scores are used for monitoring RA disease activity; US findings can be scored using quantitative or semiquantitative scoring systems to estimate the degree of synovial/tenosynovial and erosive processes.. New technologies such as elastography, three dimensional sonography, fusion imaging and contrast enhanced ultrasound can add further powerful information in the imaging of RA.