Atmospheric chemistry is an integral component of the Earth’s system, playing a role in a wide range of important processes that affect many aspects of the Earth’s environment, including the stratospheric ozone layer, air pollution, and lifetime and fate of a variety of gases and aerosols that affect the Earth’s radiative balance. CCMI works to assess the representation of atmospheric chemistry in global models, including climate and Earth system models, providing projections of how the chemical state of the atmosphere may change in the future and improving our understanding of the interactions between chemistry and climate. To make progress on these goals, CCMI regularly brings together researchers to propose a set of well-defined model experiments to be performed by participating modelling groups, with an accompanying set of diagnostic model output fields. Model data are made available for analysis in a publicly available archive. This collection brings together articles describing the analyses and evaluations of chemistry–climate model data from these sets of CCMI multi-model experiments. CCMI is jointly supported by the Atmospheric Processes and their Role in Climate (APARC) and International Global Atmospheric Chemistry (IGAC) projects.