This paper describes the requirements and constraints involved in the design of a digital scanning mammography system. The SNR model of Muntz is used to assess the influence of detector size and detected scatter-to-primary ratio on the detectability of microcalcifications in a breast phantom, and hence to consider the design of an optimized system. The effects of patient dose, exposure time and various technological constraints, such as x-ray tube power limits, maximum scanning speed and system MTF, are also discussed. It is demonstrated that the combined use of an air gap between the patient and the detector and a wider scanning slot can give the same scatter-to-primary ratio as a significantly smaller slot, while reducing the x- ray tube load. It is also demonstrated that, to some extent, the tube power constraints can be overcome by using higher kVp, less added filtration and a smaller source to patient distance.