Deer are primarily herbivores, although they occasionally have been observed sampling such incongruous foods as dead fish. Their feeding habits and preferences can vary widely from one location to another, but each local population seems to have preferred foods that are eaten first; "marginal" foods that are eaten only after the preferred foods become rare; and "starvation" foods that probably have no nutritional value, but are eaten because no other choices are available.

Deer eat an enormous variety of plants, and eat different parts of plants in all seasons. The succulent leaves of growing plants are consumed in the spring and summer, while fruits and seeds are eaten as they become available. The buds of woody plants are a mainstay of the diet in winter. Hard mast foods, such as hickory nuts and acorns, are an extremely important component of fall and early winter diets when deer, like many wild animals, need to establish fat reserves. Deer can be quite selective about certain foods, and are known to favor heavily fertilized ornamental and garden plants above others that have not been so well fertilized.

Deer are herbivores (plant eaters); most are browsers (eating leaves, shoots, soft vegetation, twigs, etc.), but some are also grazers (eating mostly grass).

Deer are ruminants; they store partly-chewed food, and later regurgitate this cud and thoroughly chew it (this is called "chewing the cud"). This process lets deer process a large amount of low nutrient food.

Deer have many predators; their main defenses are running away and hiding, although hooves, antlers, and teeth are used as a last resort. Their predators include large animals like grizzly bears, mountain lions, coyotes, and man. Small deer (like the pudu) are eaten by large birds, foxes, and small wild cats.