Companion animals and their human guardians suffer from many of the same types of cancer and are often treated with many of the same drugs. Moreover, the overall tumour biology is much more similar between humans and companion animals than between humans and rodent tumor models. Therefore, it is proposed that pre-clinical evaluation of novel cancer therapeutics should more often include appropriately designed trials in companion animals with cancer to more accurately predict efficacy and toxicity in humans. For example, studies in dogs with cancer have been used to assess efficacy and design human clinical trials of immunotherapy, gene therapy, sustained release drug delivery and liposomal drug delivery. In the future, such studies will ultimately benefit not only humans, but also companion animals with cancer.