Every aspect of the life of a black-tailed and mule deer is ultimately fueled and made possible by the nutrition they gain from their foods, which is correspondent with everything from the size and shape of their muzzle to the microflora that reside in their digestive system. Black-tailed and mule deer persist on a diverse diet of plants, most of which are shrubs and forbs that yield digestible energy and protein at levels sufficiently high to support maintenance, growth, and reproduction. Allocation of acquired resources is sensitive to the nutritional state of the animal, which is best represented by percent body fat. Through state-dependent regulation of body reserves, black-tailed and mule deer are highly fecund, most frequently pregnant with twins, and subsequently support rearing of offspring through food intake and catabolism of somatic reserves if available. Integration of measures of animal nutrition into studies of population dynamics has led to major advances in understanding population ecology of black-tailed and mule deer. We anticipate continued advances into the future along with opportunities to better communicate to a diverse public and stakeholders the nuances of the ecology and nutritional underpinnings of this highly revered species.