Cancers are a leading cause of death in humans and for many other species. Diet has often been associated with cancers, and the microbiome is an essential mediator between diet and cancers. Here we review the work on cancer and the microbiome across species. We systematically reviewed over a thousand published articles and identified links between diet, microbes and cancers in several species of mammals, birds, and flies. Some microbes, specifically Fusobacteria, Bacteroides fragilis, Helicobacter bacteria, and papillomaviruses, have cancer-inducing effects in gerbils, mice, dogs, or cats. Other microbes, such as Lactobacillus species, mostly found in milk products, prevent gastrointestinal, breast, and lung cancers in mice and rats. Future work should examine a larger variety of host species to discover new model organisms for human preclinical trials, better understand the observed variance in cancer prevalence across species, and discover which microbes and diets are associated with cancers across species. Ultimately this could help identify microbial and dietary interventions to diagnose, prevent and treat cancers in humans as well as other animals.