For at least 100 years, immunologists have proposed activating the immune system to specifically target and eradicate autologous tumor cells. The idea that tumor cells can be recognized as foreign to the host's immune system is an essential concept of tumor immunology that was first postulated by Paul Ehrlich at the turn of the century. Anecdotal reports of spontaneous tumor regression have been presumed to be immunologically mediated. With the advent of molecular gene transfer techniques and increased knowledge of the regulation of the immune response, effective methods for harnessing the immune system as a therapeutic agent are finally being realized. Current results of clinical immune/gene therapy protocols will be reviewed with consideration towards the concept of cancer prevention.