The liver is a common site of malignant disease with hepatocellular carcinoma and liver metastases accounting for the majority of liver malignancies. Conventional treatment options are confined to chemotherapy, surgery and minimally invasive procedures such as radiofrequency ablation. External beam radiation therapy has a limited role in the treatment of liver malignancy, mainly due to the substantial risk of radiation-induced liver disease. Transarterial chemoembolisation is contraindicated in patients with portal vein thrombosis or advanced liver disease. Selective internal radiation therapy using yttrium-90 microspheres has emerged as a suitable treatment of liver tumours. In this review we discuss the principles behind this treatment, the process of patient selection, and the method of dose calculation and administration. We also highlight possible adverse effects and review the literature for data on response assessment and long-term outcome.