The FOCUS method, in which both crystal chemical information and powder diffraction data are included in the structure determination process, is presented. FOCUS combines automatic Fourier recycling with a specialized topology search specific to zeolites, which can be described as having 3-dimensional, 4-connected framework structures. The capabilities of FOCUS have been tested with seven examples of medium to high complexity. The method was then applied to three novel zeolite structures and a promising model could be obtained in each case. Experience shows that the approach of using chemical and geometrical knowledge can compensate for some of the information that is lost as a result of the overlap problem. At the same time, there is an intrinsic disadvantage: any method based on assumptions of certain structural properties is also limited to materials which conform to these assumptions. Examples which show the consequences of relaxing the structural assumptions are also discussed. In...