Each year, about 6 million of the 65 million pet dogs in the United States will be diagnosed with spontaneous, naturally occurring cancer. Of the 32 million pet cats, 6 million will be diagnosed with cancer. Pet animals with naturally occurring cancers are traditionally treated by veterinary oncologists in private practices or at teaching hospitals, using therapies similar to those used in humans, including surgery, chemotherapy, radiation therapy, and biotherapy. The inclusion of this large number of animals suffering from cancer in the study of human cancer is referred to as comparative oncology. Comparative oncology brings together the work of veterinary clinical oncologists with medical oncologists, the pharmaceutical industry, and academic centers involved in cancer research. The product of this effort is an improved understanding of the biology of cancer and improved treatment options for both animals and humans afflicted with cancer.