Apoptosis is a mode of cell death that is currently of intense research interest in developmental and cancer biology. For more than 40 years, radiobiologists have been aware of cells in irradiated specimens that display the features of apoptosis. Despite this knowledge, however, it has proved difficult to elucidate the role of apoptosis in end points important in radiation therapy and to distinguish its role from that of other modes of cell death, namely, reproductive cell death. In this article, the evidence relevant to this question will be critically reviewed. Conclusions that have significant implications for radiation therapy will be drawn to suggest that tumor cells with apoptotic propensity are more sensitive to radiation. If this hypothesis is confirmed, strategies can be envisioned that may restore apoptotic propensity to radioresistant tumor cells for therapeutic benefit.