Jörn Lenz examines the rights and obligations of app stores, developers, and their users from the perspectives of civil law, competition law, and antitrust law. In this context, app stores act as intermediaries between users and app developers. The civil law classification forms the foundation of the work and distinguishes between paid, free, and apps with in-app purchases. Based on this classification, certain obligations under competition law arise, which must be fulfilled by either the app store itself or the app operator, depending on the specific arrangement. In this regard, in addition to information obligations, the advertising communications related to in-app purchases must be taken into account. Furthermore, the author also examines the position of app stores within operating systems from an antitrust perspective to understand the competitive significance of app stores. This requires defining the market based on the licensability of the operating systems, which leads to various obligations.