Virtually every modality employed in the treatment of cancer has demonstrated an adverse effect upon metastasis under some conditions. This review surveys the experimental and clinical literature pertaining to the untoward effects of ionizing radiation upon metastatic processes. Two processes are described: (1) enhancement of metastases following local tumor irradiation; (2) localization of metastasis in previously irradiated normal tissues. In the first process the experimental evidence indicates a local effect of irradiation upon the tumor-stroma interface. It predominates under conditions of non-curative radiation doses. There is no proof that this process occurs in clinical practice, but a review of data provides suggestive evidence for its existence following non-curative therapy. The second process is documented both experimentally and clinically. It requires the presence of viable, circulating tumor cells and appears mediated through vascular damage. The few clinical reports suggest that this effect is rare in practice. The clinical significance of both processes appears small under conditions of effective tumor therapy, but it is speculated that inadequate tumor irradiation, or irradiation of normal tissues with uncontrolled tumor elsewhere, may be deleterious.