This paper presents the Robotics Service Bus (RSB), a new message-oriented, event-driven middleware based on a logically unified bus with hierarchical structure. Major goals for the development of RSB were openness and scalability in order to integrate diverse components in the context of robotics and intelligent systems. This includes the ability to operate on embedded platforms as well as desktop computers, reduction of framework lock-in, and the integration with other middlewares. We describe the design of the RSB middleware and explain how it meets requirements which lead to a scalable and open middleware concept. These requirements are based on several application scenarios which are used to verify the applicability of RSB. Furthermore, we relate RSB to other middlewares in the robotics domain.