Figures-of-merit (FOMs) is a term which is used to define the quantitative measure of performance based on parameters which can be determined by the experimentalist. The FOM may be intuitive, based on previous experience of the analyst or his personal assessment of the importance of certain parameters, or derived, based on measurable parameters or on mathematically deduced data, mostly involving statistics. The FOM may also be a combination of intuitive and derived factors. In this review, a number of commonly used FOMs, due to several authors, are considered. They are categorized by their originators, their type (intuitive, derived or a combination thereof), their primary application and the factors on which they are based. The applications discussed include both x-ray spectrometry and x-ray diffraction, but many of the principles apply equally to both.