As technology advances in the communications and robotics fields and their main technologies, networked robots can offer a wide range of applications. Furthermore, most networked robotic systems consist of a collection of components (being multiple robots working together, multiple components working together within one robot or robots working with other external systems such as wireless sensor networks or servers). These components require cooperation and collaboration to achieve a common goal. However application development for such collaborative distributed systems composed of many robots with sensors, embedded computers, and human user is very difficult. Therefore, middleware services provide a novel approach offering many possibilities and drastically enhancing the development process and the overall functionalities needed for networked robotic systems. This paper surveys the current middleware characteristics in this domain. It discusses middleware challenges in networked robotic systems and presents some representative middleware solutions specifically designed for networked robots. Open issues in that domain are also discussed.