System behavior that depends so sensitively on the system's precise initial conditions that it is, in effect, unpredictable and cannot be distinguished from a random process, even though it is deterministic in a mathematical sense. This article begins with a discussion of the notions of order, chaos, and noise as they occur in deterministic dynamical systems, the relation of chaos and periodicity, and the concept of an attractor. Applications of chaos to atmospheric prediction, weather, climate, electronic circuits, astronomy, acoustics, and atoms are then discussed.