Allelochemicals are produced and released into the environment by one species and have an effect on another species. They include attractants, repellents and toxic defence compounds which could provide valuable tools for use in IPM. There have been successes with botanical pesticides such as neem and with push-pull companion crops. However, practical use of allelochemicals in IPM is still quite limited. This chapter discusses the various types of allelochemicals, constraints to their use and how these may be overcome.