Climate models are usually calibrated using present and historical climates, but projections show that future climates will lie well outside the trends and variability recorded by these references. Paleoclimate records and simulations are therefore instrumental to provide stringent tests for state-of-the-art models and to assess whether model sensitivity to forcings is compatible not only with observations of current climate but also with past climate. Simulations of five past periods are addressing the objectives of the Coupled Model Intercomparison Project (CMIP6): Last Millennium, Mid-Holocene, Last Glacial Maximum, Last Interglacial, and Mid-Piacenzian (previously, Mid-Pliocene) Warm Period. Each provides a unique perspective on the Earth system. We invite submissions that employ numerical modelling and/or reconstructions for these CMIP6 time periods. Topics could include, but not restricted to, new results on inferences of climate sensitivity and feedbacks, polar amplification and responses of sea ice and ice sheets, hydrological cycle, and interannual to centennial variability.