Cancer in an important disease throughout the world and a major cause of death in both humans and animals. Pathologists play a critical role in cancer research and in the diagnosis, treatment, and prevention of human and animal cancers. They participate in basic and translational research in laboratories of colleges and universities, other research institutes, government research and regulatory agencies, and in the biotech, chemical, and pharmaceutical industries. This introductory review to the 2009 STP Symposium on Cancer will identify and discuss the major roles of pathologists in cancer research.