All animals, even stationary ones, constantly release a stream of odorants into the air where these odorants are then carried downwind. Olfactory predators can use airborne odorant trails to detect their prey. Their ability to do so, however, varies with time and location. The sources of variation examined in this chapter are environmental variables that control the concentration of odorant molecules in the air. These environmental variables limit the ability of olfactory predators to detect the presence of prey. However, this is only the first part of an olfactory predator’s task of foraging. It must also use these olfactory cues to determine where the prey or food source is located. How it does so is the subject of Chapter 4.