StarSim is a physics-based simulation framework designed to model and correct the impact of stellar surface heterogeneity on photometric and high-resolution spectroscopic observations. As precision stellar and exoplanet characterisation increasingly relies on optical and near-infrared spectroscopy, disentangling genuine planetary signals from activity-induced imprints has become essential. In this talk, I will provide an overview of StarSim’s core capabilities and present selected applications to M dwarf science cases, including activity characterisation and the mitigation of activity-induced biases in line-profile diagnostics. Examples will illustrate how StarSim supports current observing campaigns and prepares for the coming ELT era.