One of the fundamental questions facing the emerging discipline of service design concerns the definition of its object. In this thesis, I posit that the practice of service design, as a recent development within the tradition of industrial design, may be approached primarily as the design of interfaces between service providers and clients. In chapter 1, on basis of a critical study of the service literature spanning the disciplines of management, engineering, and economics, I argue for the importance of acknowledging the materiality of interfaces when designing services. In chapter 2, I comment on relevant views in the field of industrial design about the design of (service) interfaces. Then, drawing on postphenomenological studies in the philosophy of technology, I articulate an approach to service interfaces that stresses the mediating role of materiality in client-provider relations. Chapters 3 and 4 present empirical studies of a service, called DirectLife, where digital technology plays such a mediating role. This service, which was developed and commercialized by Philips, is intended to help people become more physically active. In chapter 3, I elaborate on the user experience of DirectLife from a postphenomenological perspective, describing how its material interface transforms clients’ perceptions of their bodies and social selves. In chapter 4, turning to the provider’s perspective, I analyze the multiple visualizations generated and interpreted by the designers of DirectLife in the process of materializing a new service interface. Chapter 5 seeks to extend this postphenomenological perspective on service design beyond the scope of client-provider interactions that are mediated mainly by digital technologies. Drawing on an empirical study of a design project carried out at the Service Science Factory, I demonstrate how human-to-human interfaces may be understood from a postphenomenological perspective, and I discuss the implications of this for the design of interpersonal services. To conclude, in chapter 6 I propose that contributions of this thesis may serve to deepen the debate about the distinction between products and services and to invite designers to rethink their expertise in postindustrial times.