TOMCAT/SLIMCAT is a three-dimensional (3D) off-line chemical transport model (CTM). The model uses winds and temperatures from meteorological analyses (e.g. from the UK Met Office or ECMWF) to specify the atmospheric transport and temperatures and calculates the abundances of chemical species in the troposphere and stratosphere. The model has the option of detailed chemical scheme(s) for the stratosphere and troposphere. The model can be used to simulate the past and current atmosphere, help interpret observations, and to diagnose the extent of problems such as stratospheric ozone depletion or tropospheric pollution.