This chapter aims at obtaining as precise a definition as possible of the concept of “industry.” This is not an easy exercise because, in the traditional microeconomic approach, specialists consider either the economy as a whole, including all the goods and services and the corresponding markets, or the microcosm formed by the market of a single good for which all the units exchanged are perfect substitutes. The first approach is that of general equilibrium and the second that of partial analysis. Neither of them evokes the intuitive concept of industry. However, it is to this concept that product differentiation refers, as a set of variants of the same product whose sellers compete because these variants are imperfect substitutes for each other.