The juvenile idiopatic arthritis (JIA) is the childhood most common rheumatologic disorder. The JIA diagnosis is not based on specific clinical and serological markers, but mainly on the ultrasound and magnetic resonance imaging investigations. The use of both these imaging techniques seem to be very promising, especially for the ultrasound. In fact, the improvement in both the ultrasound methods (power e colour Doppler, high resolution technique) and the operator knowledge could give the same results of the MRI imaging leading to a better cost-benefit ratio.