Both in metamorphic and igneous petrology, equilibrium thermodynamics in combination with thermodynamic datasets and more or less well constrained solution models have been combined and used in different softwares in order to unravel at least parts of the P-T history of rocks by forward and backward modelling. In this session, the state of the art of any such software can be presented and their theoretical and practical strengths and weaknesses in application to natural problems discussed. We particularly invite field studies demonstrating the strengths or weaknesses of certain thermodynamic methods. The various aspects of the equilibrium thermodynamic approach itself could be critically reviewed, for example its application to (apparent) disequilibrium situations, the use and abuse of "pseudosections", open-system modelling etc. - all of these ideally in the form of case studies. Open issues or areas of missing data should be pointed out as directions of future research efforts. This includes the quality and availability of experimental data, state of the art experimental approaches, the quality of solution models (solids, fluids, melts)or phase/melt seggregation models - as tested in field studies and by experiments. Issues like the future of the equilibrium thermodynamic approach, its expansion towards disequilibrium modelling and how it is incorporated in larger softwares(e.g. for geodynamic modelling or "geological process modelling") can also be taken up in this session.