Smell is a duplicitous sense, simultaneously straightforward and mysterious. First it tells us the toast is burning and then it resurrects faint recollections from childhood. From our distant ancestors we have acquired hardwiring to certain smells and our response to them is reflexive but to many others we learn a response that includes the emotions, memory, and the endocrine system. The more we understand about our sense of smell, the more we have to redefine its true function. In popular belief it is a warning system: bad smells indicate spoiled food, gas leaks, and other hazards. But while the existence of bad smells is easy to explain, there is, to date, no satisfactory biological reason for good smells. It may be that our very survival depends upon our ability to distinguish between “good” genes and “bad” genes, and it appears we do this by smell.