Apoptosis and premature senescence, an acutely inducible terminal cell-cycle arrest, are known to be the ultimate cellular defense programs that counteract oncogenic transformation. Thus, activated oncogenes may sensitize cells to other stimuli that also recruit these programs. Recent evidence demonstrates that both apoptosis and premature senescence respond to drugs and can therefore contribute to the outcome of cancer therapy. However, manifest malignancies may have acquired mutations that compromise these programs at different levels, and hence may become chemoresistant to varying degrees as a result of defects in either or both programs.