Custom is an integral part of the institutional matrix of modern economies. Market processes hinge crucially on custom, which in turn affect these processes. The book proposes a theory of custom that emphasizes the motivational force that arises from the individual's striving for coherence and justification. It depicts custom as comprising habitual, cognitive, and emotional aspects and explains that market transactions rely on customary entitlements and obligations. The motivational force of custom emerges from a preference for regularity and a desire for coherence that tie cognition, emotion, and action together. The view is applied to the theory of property, the theory of the law, the theory of the firm, and the problem of the division of labour and the conditions under which that division of labour is better organized by the firm or by the market.