This chapter formalizes the supply side of economies in terms of production, apart from the initial endowments of commodities, in competitive markets. Previous chapters have developed a general equilibrium analysis that abstracts from the production possibilities open to economic agents. After introducing the product space, which is distinct from the commodity space faced by individual agents, production is defined as technological transformations in the product space. After discussing two types of production, namely Hicksian production and Walrasian production, the chapter proceeds with the general equilibrium analysis of perfectly competitive markets with Walrasian production.