Restenosis following PTCA or intracoronary stent insertion remains the greatest challenge to interventional cardiology. Intracoronary brachytherapy may use either beta- or gamma- radiation. The target cells are most likely in the adventitial layer of the vessel wall. The principle of using brachytherapy post-angioplasty to reduce restenosis has been proven in animal models. Multiple randomised trials have shown brachytherapy to be the current optimal therapy to treat in-stent restenosis. The data for the use of intracoronary radiation for treatment of de novo coronary lesions are less strong. Potential complications of brachytherapy include ‘edge effect’ and ‘late late stent thrombosis’. These problems are being minimised with the use of long sources and prolonged antiplatelet therapy. Drug delivery stents may challenge the role of brachytherapy in preventing and treating restenosis in the near future.