The European Commission's Communication provides guidance on defining the relevant market for EU competition law purposes. The Communication aims to improve transparency on the application of market definition in competition proceedings. It sets out a general methodology for defining relevant product and geographic markets, in particular with regard to demand and supply substitutability. It also deals with specific cases of market definition, such as significant product differentiation, multi-stage platforms and innovative competition. Finally, the Communication addresses the role of market shares in the assessment of market power.