Why do repellents repel?

Repellents are used by humans and by animals to protect themselves from bites, and they contribute controlling the transmission of vector-borne diseases. There are numerous chemical formulations of both natural and synthetic origins, as well as electronic devices, that are commercially available worldwide alleged to provide personal protection against insect bites. Not all insect repellents are effective enough to provide real protection, nor are they considered safe for humans, animals or the environment. There is an urgent need to find novel, nontoxic and environmentally friendly alternatives. However, current efforts in this direction face a major obstacle: our understanding of how they keep away mosquitoes and other biting arthropods remains incomplete. This is also true for DEET, the ‘gold standard’ of repellents and the most widely used since its discovery in the 1940s. This article aims to guide the search for new repellents by discussing our current knowledge and future needs.