Detecting and discriminating various chemicals in the environment is critical for the survival of most animals on this planet. They depend on their sense of smell (olfaction) to locate food, mates, and predators. For a compound to be smelled by air-breathing animals, it needs to be volatile or semivolatile. In general, odors are nonionic, hydrophobic, organic molecules with molecular weights of no more than 350 daltons. The odor quality of a molecule is largely determined by its size, shape, and functional groups. Mammals are able to distinguish among thousands of odors, and this amazing task is performed by the olfactory system.