This research aims to investigate how building energy simulation tools are used in the first design stages, focusing on the design practice of architects willing to obtain the optimal energy performance in order to create sustainable architecture. We survey in which measure the simulation tools could guide the choices of the designer, if they are used by consultants or by the designers themselves, and which are the main reasons that prevent their use in the common design practice. The research is conduct through these instruments: literature research, research by professional figures-questionanaires-,research through a case study.