A pest is any animal or plant harmful to humans or human concerns. The term is particularly used for creatures that damage crops, livestock, and forestry or cause a nuisance to people, especially in their homes. Humans have modified the environment for their own purposes and are intolerant of other creatures occupying the same space when their activities impact adversely on human objectives. Thus, an elephant is unobjectionable in its natural habitat but a pest when it tramples crops.

Some animals are disliked because they bite or sting; snakes, wasps, ants, bed bugs, fleas and ticks belong in this category. Others enter the home; these include houseflies, which land on and contaminate food, beetles, which tunnel into the woodwork, and other animals that scuttle about on the floor at night, like cockroaches, which are often associated with unsanitary conditions.

Agricultural and horticultural crops are attacked by a wide variety of pests, the most important being insects, mites, nematodes and gastropod molluscs. The damage they do results both from the direct injury they cause to the plants and from the indirect consequences of the fungal, bacterial or viral infections they transmit. Plants have their own defences against these attacks but these may be overwhelmed, especially in habitats where the plants are already stressed, or where the pests have been accidentally introduced and may have no natural enemies. The pests affecting trees are predominantly insects, and many of these have also been introduced inadvertently and lack natural enemies, and some have transmitted novel fungal diseases with devastating results.

Humans have traditionally performed pest control in agriculture and forestry by the use of pesticides; however, other methods exist such as mechanical control, and recently developed biological controls.