  Chemical kinetics plays an important role in controlling the atmospheric
composition of all planetary atmospheres, including those of extrasolar
planets. For the hottest exoplanets, the composition can closely follow
thermochemical-equilibrium predictions, at least in the visible and infrared
photosphere at dayside (eclipse) conditions. However, for atmospheric
temperatures < ~2000 K, and in the uppermost atmosphere at any temperature,
chemical kinetics matters. The two key mechanisms by which kinetic processes
drive an exoplanet atmosphere out of equilibrium are photochemistry and
transport-induced quenching. We review these disequilibrium processes in
detail, discuss observational consequences, and examine some of the current
evidence for kinetic processes on extrasolar planets.
