In 2008 the temporal focus of the Palaeoclimate Modelling Intercomparison Project was expanded to include a model intercomparison for the mid-Pliocene warm period (ca. 3 Ma BP). This project is referred to as PlioMIP (Pliocene Model Intercomparison Project). Two experiments have been agreed upon and comprise phase 1 of the PlioMIP. The first (Experiment 1) will be performed with atmosphere-only GCMs. The second (Experiment 2) will utilise fully coupled ocean-atmosphere GCMs. In previous PMIP-style studies it has often been challenging to disentangle the effects of differences in model parametrisations and physics from differences in the methodology employed to implement the palaeo-boundary conditions. This special issue of Geoscientific Model Development provides a means for each participating group within PlioMIP to provide detailed information on how Pliocene boundary conditions were included within their models. This will help facilitate the identification of discrepancies between models at the intercomparison phase. The issue also provides a means for each group to provide basic climatology's from each PlioMIP experiment, ensuring that a minimum level of output is readily available for the intercomparison phase. Finally the issue includes two papers that describe in detail the experimental design and boundary conditions used for both PlioMIP Experiments 1 and 2.