The “Theory and Simulation of Solar System Plasmas” session solicits presentations of the latest results from theoretical investigations and numerical simulations in space plasma-physics from microscopic to global scales, in comparison with experiments and observations in the heliosphere: at the Sun, in the solar corona, in interplanetary space and in planetary magnetospheres. There are challenging questions in fundamental solar system plasma physics which require the analyses of huge amounts of data, in particular of the particle kinetics. Machine learning techniques have to be used. We further encourage presentations of theory and simulation results relevant to current, forthcoming and proposed space missions. Each year a topic of special interest is chosen as a focus of the session. For 2021 this focus will be on synergies between observations in the solar wind made by Solar Orbiter and Parker Solar Probe with theory and simulation.