Radiotherapy has historically played a minor role in the treatment of patients with unresectable liver metastases from colorectal cancer and other malignancies. This can be attributed chiefly to the low tolerance of the whole liver to radiation. High-precision radiotherapy planning techniques have allowed much higher doses of radiation to be delivered safely to focal liver metastases, while sparing most of the normal liver. When combined with hepatic arterial fluorodeoxyuridine, high-dose focal liver radiotherapy is associated with excellent response rates, local control, and survival in patients with unresectable liver metastases from colorectal cancer. Radiotherapy, with and without concurrent systemic chemotherapy, has also been used with encouraging outcomes for patients with liver metastases from colorectal cancer and other cancers. There appears to be a radiation dose response for liver metastases; tumors treated with doses of 70 Gy or greater are likelier to have durable local control. Advancements in tumor imaging, in radiotherapy techniques that will allow the safe delivery of higher doses of radiation, and in novel tumor radiation sensitizers and normal tissue radioprotectors should substantially improve the outcome of patients with unresectable liver metastases treated with radiotherapy.