The dogma that antineoplastic treatments kill tumour cells by damaging essential biological functions has been countered by the notion that treatment itself initiates a programmed cellular response. This response often produces the morphological features of apoptosis and is determined by a network of proliferation and survival genes, some of which are differentially expressed in normal and malignant cells. Correspondingly, mutations that interfere with the initiation or execution of apoptosis may produce tumour-cell drug resistance. Remarkably, many of the genes that modulate apoptosis in response to cytotoxic drugs also affect apoptosis during tumour development; hence, the process of apoptosis provides a conceptual framework for understanding how cancer genes can influence the outcome of cancer therapy. Although the relative contribution of apoptosis to radiation and drug-induced cell death remains controversial, clinical studies have associated anti-apoptotic mutations with treatment failure. While careful preclinical and clinical studies will be necessary to resolve this point, our current understanding of apoptosis should facilitate the design of rational new therapies.