The proper definition of the relevant product market still is the lynchpin of competition law: defining the market too wide makes it impossible to capture the companies' behavioural margins that are the result of market power and are not neutralized by competition; defining the market too narrow creates market power and forces undertakings under the application of Art. 82 EC, Art. 2 ECMR, § 19 German Competition Act. In European and German competition law the aspect of demand-side substitutability has been the most important criterion. The meaning of potential competition and especially of supply-side substitutability has not been systematically grasped. The book therefore re-thinks the relevant questions of proper market definition.