What would the environmental benefits be, if the world’s dogs, cats and people all consumed vegan diets, and how many additional people or pets could be fed with the food energy saved? How much livestock consumption is due to pet food? After analysing pet food ingredients in detail, as well as food consumed by people, I was able to calculate these benefits. They are game-changingly large. Vegan diets for dogs, cats and people, would save many billions of livestock animals from slaughter annually, would eliminate vast quantities of greenhouse gases, pesticides and other damaging emissions, would save vast amounts of land (allowing reforestation) and freshwater. Recent very large-scale studies have shown health outcomes as good or better for both cats and dogs fed vegan diets (see www.sustainablepetfood.info > health). And studies of feeding behaviour have demonstrated that average dogs and cats enjoy vegan pet foods as much as those made from meat. Hence, pet owners who care about the environment or their animals’ health should consider nutritionally-sound vegan pet food. However, to safeguard health, people should feed only diets labelled as nutritionally complete, produced by reputable companies with good standards.