Clinical trials for companion animals are becoming more common and more accessible to pet owners as veterinary oncologists seek to expand their knowledge of tumor biology in companion animal species and improve the way they diagnose and treat cancer for these animals. Many owners enroll their pets because they wish to participate in clinical cancer research that may ultimately benefit pets and people. Understanding the goals, benefits, and risks of clinical trials participation provides the knowledge needed by primary care veterinarians to counsel their clients as to whether clinical trial participation is a good choice for them and their pets.