Just in time for the 25th anniversary a new version of the international well-known simulation tool TRNSYS is released: TRNSYS 15. New developments of both user-friendly interfaces and mathematical models have been implemented responding to a growing commercial application of TRNSYS. For example, the graphical input interface IISiBat has been completely rewritten in C++ and offers now all the comfortable features of modern window programs. In addition, suggestions of users such as layer techniques, reading ASCII-Input files and improved project and component management and exchange are realised. Also, the multi-zone building model has been improved significantly. A new model for the simulation of a thermally activated slab for heating / cooling is integrated into the building model. In addition, comfort evaluation is incorporated in the standard version. The improvements of the solar radiation distribution within a zone and through an internal window into an adjacent zone yield to a significant reduction of the effort of modelling double facades and air flow windows. The flexibility and openness of TRNSYS 15 is increased by the ability to call external programs and external DLL’s (Dynamic Link Libraries). Thereby, components which are not programmed in Fortran and/or not specially customised for TRNSYS can be easily added. Due to these new features TRNSYS is an even more powerful tool for the thermal simulation of buildings and energy systems that satisfies new demands of innovative and forward-looking energy concepts.