In its quest to improve well-being and safety in buildings, CSTB plies four complementary trades: research, advanced engineering, quality assessment and the dissemination of knowledge. In combination with its fields of expertise, they allow CSTB to adopt a global approach to buildings which includes their urban environment, services and the new information and communication technologies. CSTB provides software solutions. On the solar energy field, CSTB together with AIGUASOL, have developed and launched TRANSOL 2.1. TRANSOL 2.1 focuses on Solar Thermal Systems and brings dynamic calculation accuracy of TRNSYS 16 (TRANsient simulation SYStems) closer to users. Within a user-friendly interface, designers can choose between a wide range of solar thermal schemes (including apartment blocks), for different applications (DHW, Heating, Pool) and run simulations, with different parameters to allow optimisation of orientation and inclination of solar panels, tank volumes, insulation, flows,… with the powerful TRNSYS engine. This software includes meteorological data for 500 locations in Europe.