Repellents generally work by taking advantage of an animal's natural aversion to something, and often the thing chosen is something that the animal has learned to avoid (or instinctively avoids) in its natural environment.
For example, some animals will avoid anything that has the odor of the urine of certain predators. Tiger urine is thus very effective at keeping away animals. Coyote urine has gained currency as a deer repellent. Fox urine is used to repel rabbits, groundhogs, woodchucks, squirrels and chipmunks. Bobcat urine repels moles, mice, voles and other rodents. Wolf urine is used to repel moose.
Chemical repellents mimic natural substances that repel or deter animals, or they are designed to be so irritating to a specific animal or type of animal that the targeted animal will avoid the protected object or area. Some chemical repellents combine both principles. There are many homemade deer repellent recipes on the web.
For example, the lawn fertilizer Milorganite is claimed to be an effective repellent due to its smell. Repellents fall into two main categories, odor and taste. Odor repellents work better in the warm seasons and taste repellents work better in the cold months. Taste repellents only work after the deer or other animal has taken a bit out of the plant. If you have a plant you don't want nibbled on at all, use an odor repellent or a combination of both.
Other types of non-chemical repellents are sometimes used. A simple electrified or barbed wire fence can mechanically repel livestock or predator animals. Some electrical repellent systems have been tested against sharks. High-frequency whistles have been used on vehicles to drive deer away from highways, and similar devices have been used to deter and repel certain types of insects or rodents. Repellents for domestic cats and dogs can also be found; these include ultrasonic devices which emit a high frequency noise that does not affect humans. These types of non-chemical repellents are quite controversial because their effectiveness varies from person to person. Furthermore, there have been few scientific studies conducted to prove that they do work. They are, however, a safe and humane way of disposing pests.
There are also few alternate ways to repel Cats and Dogs which are effective, such as repellent chemical Spray, electronic pet barrier, motion activated sprinkler etc. In Addition, vinegar can be used as organic pet repellent.
 Flashing lights are used to repel lions in Kenya.
The ideal repellent is completely specific for the target animal; that is, it drives away the animal that one wishes to repel without affecting or harming any other animals or people. One type of animal repellent may be effective for raccoons, while another animal repellent may be more effective for skunks. It can be difficult to design a repellent method that drives away only undesirable animals while having no effect on people or other creatures.
Some animals are more likely to be targeted than others by human users of repellents. Targeted animals are often predators of animals of interest to human beings, such as food fish or livestock. Sometimes the targeted animals are predators of human beings themselves.
Research has shown that cinnamon oil, clove oil, and eugenol are effective snake repellents. Snakes will retreat when sprayed directly with these oils and will exit cargo or other confined spaces when these oils are introduced to the area.
In ancient times the Greek historian Herodotus noted that Arabian harvesters of frankincense used burning resin from Styrax trees to repel poisonous snakes that lived in the trees.
Moth balls
The roots and other parts of Acacia polyacantha subsp. campylacantha emit chemical compounds that repel animals including rats, snakes and crocodiles. For snakes, roots are placed in the rafters of houses.