Whereas apoptosis is a critical mode of cell deletion in normal organism development, apoptotic cells are also observed in tumors, especially following cytotoxic treatments, leading to questions about their role in tumor response to therapy. We have conducted a series of studies using murine tumor models and found that the ability of the tumor cells to undergo apoptosis correlates with tumor response to radiation. The best correlation was with the pretreatment apoptotic index, suggesting that apoptosis in some tumors may govern radiocurability by regulating the number of tumor clonogens. However, other roles for apoptosis in tumor response to radiation have not been ruled out. One of the important observations that has come from this work has been the heterogeneity in apoptosis propensity both within the cell population of a given tumor and among different types of tumors. Such findings underscore the fact that apoptosis is under complex genetic control and that some of the same oncogenes and tumor suppressor genes that are responsible for tumor initiation and progression to malignancy also dictate the apoptotic response to treatment. Understanding the biochemical and molecular pathways that govern this process may ultimately allow the development of strategies for modulating apoptosis for therapeutic benefit.