The validity and importance of the immunosurveillance of tumors have been debated ever since Burnet and Thomas outlined several key concepts that underpin our understanding of tumor immunity. With the development of gene-targeting technology, it has been shown that interferon gamma (IFN-gamma) and perforin--key effector molecules of both the innate and adaptive immune systems--contribute to the prevention of of tumors in mice. These findings have rejuvenated interest in the immunosurveillance of tumors, and recent data have provided evidence that the immune system influences the development of some spontaneous epithelial malignancies.