This chapter traces the evolution of chemical communication in animals and considers how mammals, and humans in particular, use odours in their daily activities. Comparative anatomy provides a record of how evolutionary processes have moulded tissues and organs, just as the Antarctic ice core provides a clear record of past climates, or mitochondrial DNA records the evolutionary past of a species. The science of comparative anatomy of animals teaches important facts about the common origin of the olfactory system and the anterior part of the pituitary. The chapter examines the pheromones, the chemical messengers that pass between individuals much as hormones pass messages within the individual's body. It outlines some parallels between the nose and the immune system, since both systems function to discriminate between the internal and external milieus of an organism. Finally, the chapter describes the controversial vomeronasal system, which is the secondary olfactory system.