This evaluation guideline describes the criteria to be satisfied by reaction mechanisms for modeling the chemical reaction dynamics of trace gases in the troposhere. For the greater part of the applications, the chemical reaction mechanism will have to be combined with a meteorological dispersion model. The combination of the chemical and meteorological schemes is exemplified by three model types pursuing different objectives and differing in the magnitude of the computational index. Furthermore, examples of the major applications of combined chemical and transport models are given.