The purpose of this paper is to examine the definitional foundations of the empirical and theoretical analyses which use the concept of the Urban Informal Economy. The author discusses in detail the lines that have to be drawn around the concept. She first looks at the border with that which is not Urban, then at that which is not Economic, and finally with that which is not Formal. The practical difficulties in setting these boundaries and some of the implications for analysis of having done so are discussed and a series of reminders are issued of what tends to get left out. The author then turns to the motley creature that is nevertheless left inside and makes a few illustrative references to how theorising, empirical studies and policy-making might take its complexity and diversity into account. The conclusions are summarised at the end. Where appropriate the author has tried to illustrate her arguments with references to empirical studies, mostly the two on which she herself has worked - Bombay and Abidjan.