Our sense of smell, as Trygg Engen reminds us early in this definitive work, has been neglected as a research area. This neglect belies the very critical role that the sense plays in human adaptation to the environment through the monitoring of odors. Smell is learned through experience and results, Engen maintains, in a schema of memory system that enables individuals to process and categorize odors. There are closer relationships between the individual detecting an odor, the circumstances or environment, and the reaction of pleasure or aversion than with the other senses. When future...