Simple SummaryCompanion animals with cancer have participated in veterinary radiation oncology research. This research has led to greater understanding of radiation safety and toxicities. However, the authors predict an even greater contribution from companion animals in the future. For the past forty years, technology in veterinary radiation oncology has progressed tremendously. Moreover, education and organization within the veterinary field have become more similar to human radiation oncology. These developments enable a greater contribution to translational research. In addition, a great advantage of veterinary research is that companion animals present with spontaneous cancers, including tumors with genetic heterogeneity and intact immune systems, making them similar to human cancers. The authors predict this advantage, along with the recent evolution in the veterinary field, will make companion animals key contributors to future discoveries in radiation oncology.AbstractMany successful preclinical findings fail to be replicated during translation to human studies. This leads to significant resources being spent on large clinical trials, and in some cases, promising therapeutics not being pursued due to the high costs of clinical translation. These translational failures emphasize the need for improved preclinical models of human cancer so that there is a higher probability of successful clinical translation. Companion-animal cancers offer a potential solution. These cancers are more similar to human cancer than other preclinical models, with a natural evolution over time, genetic alterations, intact immune system, and a permanent adaptation to the microenvironment. These advantages have led pioneers in veterinary radiation oncology to aid human medicine by elucidating basic principles of radiation biology. More recently, the veterinary and human radiation oncology fields have increasingly collaborated to achieve advancements in education, radiotherapy techniques, and trial networks. This review describes these advancements, including significant prior research findings and the evolution of the veterinary radiation oncology discipline. It concludes by describing how companion-animal models can help shape the future of human radiotherapy. Taken as a whole, this review suggests companion-animal cancers may become widely used for preclinical radiotherapy research.