Part I of this book critically assesses the distinctive conception of the firm that prevails in competition law, in light of issues of contemporary relevance. Chapter 3 introduces such a conception, reflected in the concept of undertaking, and in the single entity doctrine. In the EU, the concept of undertaking, defined as any entity engaged in economic activity, determines the substantive reach of competition law. In the EU and in the US, the single entity doctrine supports the idea that relationships among entities forming part of a single entity should be immune from the prohibitions on anticompetitive agreements. The doctrine is thereby used to distinguish an undertaking from the other, and to ascertain whether entities will be subject to competition law either separately or jointly. In the EU, the doctrine is also used to attribute liability in case of undertakings composed by economically affiliated but legally independent companies. Theoretical concepts used to define the boundaries of the firm may also be used for determining all activities which should be left outside the realm of competition law: non-economic activities and matters that relate to the internal organisation of companies. However, attribution of liability demands an alternative framework, based on the agency theory.