Radiosynovectomy is a safe and repeatable treatment method of chronic synovitis with synovial overgrowth and refractory chronic or acute inflammatory joint effusion. It consist in the intraarticular administration of a radioactive isotope in the form of a colloid causing the extinguishing of active synovitis. The radiocolloid causes permanent irradiation of the synovium with beta ray electron beams, which ultimately leads to its fibrosis and extinguishes the inflammatory process destroying the joint. The main indications for radiosynovectomy include chronic and acute arthritis in the course of systemic diseases, intraarticular bleeding in hemorrhagic diatheses (hemophilia), selected cases of osteoarthritis, recurrent effusions following surgery, e.g. arthroplasty, or other iatrogenic post-surgery complications causing arthritis. Radiosynovectomy is also performed in pigmented villonodular synovitis and crystal synovitis. The most common method used to determine the eligibility for radiosynovectomy is an ultrasound, which shows the location and activity of the thickened synovium. The administration of a radiocolloid into the joint, sheath or bursa should also be performed under the control of the ultrasound image, as this ensures a precise location of the puncture needle and full control of the isotope administration process. Clinical efficacy of radiosynovectomy depends on the proper qualification of patients for the procedure. The success rate of radiosynovectomy in common indications is 65–80%. It is confirmed by the visualization of avascular (fibrotic) synovium in follow-up ultrasound tests. The aim of this article is to present techniques and indications for the radiosynovectomy treatment.