This article deals with the evaluation of chemical kinetics and photochemical data for use in atmospheric modeling. Models used to calculate the temporal and spatial variation of atmospheric trace gas composition are based on chemical schemes which require chemical kinetics data for the elementary reactions involved. These data derive from careful experimentation under conditions relevant for the atmosphere, carried out in laboratories worldwide using ever improving technology over the past 50 years. The article contains an account of the background issues of atmospheric composition change which have stimulated this branch of atmospheric science; this is followed by a section on parameterisation and evaluation of kinetics data. A description of currently available on-line databases is provided. The final section contains a short description of some recent significant improvement in knowledge of rate constants for key reactions of interest in stratospheric and tropospheric chemistry.