The CSIRO Mk3L climate system model is a computationally-efficient coupled general circulation model, designed primarily for the study of climate variability and change on millennial timescales. The model distribution is freely available to the research community. This Special Issue allows the history and evolution of Mk3L to be comprehensively documented within a single issue of a journal. Papers are welcome which describe and evaluate components of the model, and which describe and evaluate subsequent enhancements to the modelling system. The issue is also intended for papers which document specific experiments, particularly those which contribute towards Model Intercomparison Projects.