An integral part of the treatment of most thyroid cancer patients is the administration of 131iodine. These patients should be hospitalized when given 131I. This paper reviews the basics of nuclear and radiation physics as applied to 131I therapy. Discussed in detail are the various facets of radiation safety which nurses need to be aware of when caring for a thyroid cancer patient treated with 131l. Among these facets are patient room assignment, regulation of visitors, patient feeding, handling of wastes, and procedures to follow in case of emergency surgery or death.