The literature on logistics outsourcing relationships frequently mentions the risk that a customer will become dependent on its 3PL provider. However, research into this theoretically and practically relevant topic remains scarce. This article examines the emergence of dependence and lock-in effects in inter-organizational relationships by analyzing the example of a third-party logistics relationship. Using a grounded theory approach in a real-life case between a German mechanical engineering company and its service provider, we found four mechanisms (convincing, tying, complementing, and locking-in) that explain dependence and lock-in effects in 3PL relationships. Based on our empirical findings, we developed a grounded model of factors and mechanisms that lead to dependence and lock-in situations in logistics outsourcing relationships. Our model makes it possible to derive implications for theory and managerial practice, as well as starting points for future research.