This chapter explores natural pest control agents of practical or historical importance. Natural products have been used to control animal pests, plant diseases, and weeds since ancient times. The biological activity spectrum of natural pesticides is extremely variable, and the selectivity is often achieved by targeted application, often relying on the behavior of the target pest. However, the biological property of a chemical is a function of its structure rather than its origin. The biological activity exerted by a given dose of the compound under given circumstances, especially as related to safety, depends on the way in which the chemical is used. Natural substances used for pest and disease control are generally nonpersistent under field conditions. Most of these often-complex molecules are readily transformed abiotically by light and/or oxygen into less toxic products. There are many natural insect control agents, most of which were discovered by empirical screening of plants and, recently, other natural sources. Moreover, research on insect physiology and behavior made possible the commercialization of chemicals that can be used to manipulate insect development or behavior.