The Coupled Model Intercomparison Project (CMIP) has been a major, very successful endeavor of the climate community for understanding past climate changes and for making projections and uncertainty estimates of the future in a multi-model framework. CMIP has developed in phases, with the simulations of the fifth phase, CMIP5, mostly completed. This special issue describes the new design and organization of CMIP and the suite of experiments of its next phase (i.e., CMIP6) in a series of invited contributions. The description of the experiments and forcing data sets presented here define CMIP6 in detail. The papers provide the required information to produce a consistent set of climate model simulations that can be scientifically exploited to address the three broad scientific questions of CMIP6: (1) How does the Earth system respond to forcing?, (2) What are the origins and consequences of systematic model biases?, and (3) How can we assess future climate changes given climate variability, predictability and uncertainties in scenarios? The special issue will include an overview paper on the CMIP6 design and organization, contributions from CMIP6-endorsed MIPs and descriptions of the forcing data sets.