Mammalian herbivores are responsible for significant damage that may total billions of dollars to a variety of agricultural products, including crops, landscape ornamentals, and timber. The challenge of the resource manager is to minimize the conflict between herbivore and human activity while recognizing that consuming plant material is the herbivore's function in the ecosystem. Diet selection by herbivores is the product of many distinct behaviors occurring in concert. Among these are behaviors that do not arise directly from plant-animal interaction; e.g. predator avoidance, habitat selection, competition, territoriality, etc. However, diet selection is strongly influenced by the interaction of the consequences of consumption (or prior consumption) and the flavor of a food, i.e. palatability. In other words, herbivory is strongly influenced by the interaction of plant chemistry and herbivore processing of phytochemicals. Likewise, herbivore responses to repellents are strongly influenced by the repellent agent and the physiological responses these agents produce in the herbivore. A number of deterrents have been employed for the protection of agricultural resources. Guard animals, exclosures, supplemental feeding, and a laundry list of chemicals (including animal products such as egg, blood, urine, etc.) have been tested and incorporated into tools intended to repel herbivores from select resources. This article focuses on the modes of action of these chemical repellents. Current research suggests that mammalian herbivore repellents promote feeding avoidance behavior via four mechanisms: 1) neophobia; 2) irritation; 3) conditioned aversion; and 4) flavor modification. These mechanisms are employed singly or in combination in numerous commercial formulations. Many chemical repellents registered for use in the United States are applied directly to plant tissues (i.e. "contact repellents"). To the herbivore, these repellents are a collection of taste, odor, visual and tactile cues much in the same way plants are characterized by these sensory qualities. Furthermore, the sensory cues of repellents are associated with the post-ingestive consequences of their consumption – precisely in the same manner that plant cues and consequences are integrated by the herbivores. Diet selection as it underlies interactions of herbivores with plants and repellent agents is reviewed. For discussion, production crops as those planted on a rotational basis with expectation of an economic return are defined. This includes the fields of agriculture, agronomy, agro-forestry, horticulture, silviculture, and viticulture. Further, repellents as products applied to crop systems for the chief purpose of reducing damage caused by foraging activities are defined.