HighResMIP is a new protocol for the Coupled Model Intercomparison Project Phase 6 (CMIP6) with the aim of furthering our understanding of model bias through horizontal resolution changes. It specifies an experimental setup which constrains models to be more comparable through simplified forcing and using only the physical aspects of climate models. This enables a cleaner comparison of the impact of model horizontal resolution on simulation performance, with each model being run at two resolutions: one typical of CMIP6 DECK simulations, and one towards 25 km resolution (atmosphere and ocean). The special issue will bring together comparable model descriptions, as well as describing the implementation of the HighResMIP protocol and assessing its strengths and weaknesses.