In 2016, the German research programme "Urban Climate Under Change [UC]²" was launched by the German Federal Ministry of Education and Research. The major goal of this programme has been the development of a new urban climate model that allows simulations that are building-resolving for entire cities and that cover an area of up to 2000 km². In this framework, the established open-source large-eddy simulation model PALM (at version 4.0) was chosen as the model core, and missing components relevant to simulate urban boundary layers and thus urban climate have been added and now form the new PALM model system. The PALM model system now consists of PALM and new components for urban applications, which are referred to as PALM-4U components. This special issue covers the model description of the PALM model system, i.e. PALM (version 6.0) including PALM-4U components as well as the description of first numerical experiments conducted with the PALM-4U components, including evaluations against wind tunnel and field observations.