Comparative oncology is an approach to understanding, diagnosing, and treating cancer using animals, particularly dogs, as models for human diseases. Many canine cancers share features with human cancers and vice versa. We also share the same environmental risks for cancer. As a result, what helps dogs often helps humans and vice versa. As veterinary oncologists and human oncologists work together on cancer, we see more and more clinical trials designed to discover what helps both species. Dogs are the preferred animal for human cancer trials for this reason.