The purpose of this chapter is to provide an outline of some prominent issues in the area. There is particular emphasis upon marketing considerations and applications. The marketing man can be faced nowadays with bewildering volumes of data, gathered from mammoth surveys, and analysed by highly abstruse techniques. There is a danger of both marketing people and researchers becoming lost in a forest of complexity. In the present chapter it is hoped to illustrate how even the most sophisticated techniques can have clear and practical applications for both long-term planning and day-to-day decisions. Given the generality of the segmentation concept, some of the points discussed below are also dealt with elsewhere in the book, usually in greater detail.