From an evolutionary perspective, chemical sensing is perhaps the most ancient of biological senses not dependent on mechanical touch. In the original aquatic environment, it was a form of remote sensing as even single-celled organisms could/can “taste” compounds in water. Such a sense was/is essential for the detection of food and the avoidance of chemical hazards. In this way even simple animals can remove themselves from an environment too low in salt, or too high in salt. It is important to distinguish between the sensing of something chemical for the bene‹t of the whole organism and the use of chemical receptors for internal processes. With the evolution of multicellular organisms (which allowed for the partitioning of the internal and external environment on a larger scale, and for tissue/organ specialization), the basic concept of chemical sensing was internalized, as well as remaining in its external locations. The creation of tissues meant the specialization of dedicated speci‹c receptor systems; more correctly sensors. Once the aquatic environment was left, external sensors on the surface of the entire organism became two important variants of chemical detection: olfaction (smell) and gustation (taste). Over the same evolutionary time period, the processor for handling the information needed to develop as well. The simple organisms reacted to the sensed molecules: they either moved towards the source and engulfed it (food), or they moved away from it (not food, possible threat). As the organisms became more complex, they had more complex behaviors and systems to process, requiring a “brain.” Once the brain and the chemical sensors were inextricably linked the rest of the evolutionary story is one of continual development of the processing and use of chemical sense information. The chemical senses become exquisite in their detection and provide some remarkable adaptations for certain animals.