The many different parties that influence design and control decisions for buildings, such as building owners, users, architects, contractors, etc, may have different and to some extent contradicting requirements to buildings. Furthermore, national building regulations specify requirements to (among others) the energy performance and quality of the indoor climate, which also must be satisfied. This paper describes a method intended for estimating design and control decisions that satisfy the given requirements, and that at the same time are optimal in some sense, for instance with respect to the economy, energy performance, or indoor climate. The method addresses this issue by combining building simulation methods with numerical optimization methods. The paper describes the problem formulation that the method is based on, the numerical simulation and optimization methods used for solving the problem, and results obtained with the method. Future development of the method is briefly discussed. Key words: Building simulation, building economy, energy performance, indoor climate, numerical optimization.