Improving existing greenhouse structures in terms of insulation and other features can save energy with significantly lower investment costs than building new greenhouses. Within the EU Framework VI project GREENERGY a decision support system has been developed that offers the potential to be used by the advisory services for growers all over Europe. It evaluates the impacts of either using different greenhouse materials (e.g. for the cover or screens) or building a complete new structure on the overall energy consumption and crop yield. The system is constructed as an easy to use software tool based upon a set of simulation model modules for greenhouse energy fluxes, crop growth and yield. The user defines the structure of his present greenhouse from the pre-defined menu. This greenhouse is then used as reference greenhouse that can be compared to the various modifications of it that the user selects. The entering of additional variables such as geographic location (country), type of crop, internal greenhouse climate set points etc. allow simulations of energy consumption and crop yield over a period of one year using reference climate data of the respective location as input to be performed. The system is constructed in a way such that the database of greenhouse construction materials can be updated easily to maintain its applicability.