Hot work tool steels are exposed to high temperatures and are additionally mechanically loaded. Suchlike cyclic load spectra with overlapping of thermal and mechanical loads cause different damage mechanisms, as caused by only time dependent creep or time independent fatigue mechanisms. This literature survey tasks with the microstructure of hot work tool steels in general, the change of the same at thermo-mechanical loads, and the resulting damage mechanisms. Using microstructure models, based on metal physics as well as plasticity models, damage can be described and lifetime, i.e. cycles to failure, can be calculated. Different approaches of microstructure and damage modelling as well as simulation, like crystal plasticity, cellular automata and the Monte Carlo method, are shown. Forward looking, in a PhD-thesis, hardening mechanisms, connected with an increase of dislocation density and softening processes, which influence the material properties, are going to be modelled by microplane and crystal plasticity methods.