The successful completion of each stage in the multistep transformation of a normal cell to malignancy involves a number of intrinsically unlikely events. A limited number of target genes, either proto-oncogenes or tumour suppressor genes, require to be mutated in the appropriate manner to confer a selective growth advantage on the target cell. Such mutational events can either take place spontaneously or they may be induced by exposure to chemical or physical mutagens. In the latter case, the cell must escape the direct toxic effects of the carcinogenic agent and in some way circumvent its own attempts to enter a suicide pathway, which is a frequent primary response to DNA-damaging agents. All of this must, of course, take place within the correct target cell, i.e., either a stem cell with a naturally extended lifespan, or a more committed cell which can acquire an extended lifespan upon exposure to a carcinogenic stimulus.