Several predictors of tumor radiocurability are already integrated into clinical practice, for example, tumor size, gross morphology (that is, infiltrative or exophytic), histologic type and grade. These are nonspecific and relatively imprecise. The aim of research into predictive assays is not only to refine the discrimination of existing predictors, but also to suggest specific experimental approaches for overcoming tumor radioresistance in individual patients. Two broad categories of predictive assays can be defined: direct and indirect measurement of tumor cell survival and/or repair capability following irradiation, and measurement of cellular and extracellular parameters affecting radiosensitivity. The ongoing research at The University of Texas M. D. Anderson Hospital is overviewed to illustrate potential methods for predicting radiocurability.