WorkPartner is the prototype of a lightweight human-like mobile service robot designed to work interactively with humans in an outdoor environment. The tasks are similar to those a human can do and the robot may replace or work together with him/her. Mobility is based on a hybrid system, which combines the benefits of both legs and wheels to provide good terrain negotiating capability and large velocity range on variable ground. The robot is powered by a hybrid power system with electrical actuating and energy storage in the form of gasoline. The robot is equipped with a human-like two-hand manipulator. The long-term goal is to develop an adaptive and learning robot, which can carry different tools and work interactively with humans. The user or operator can be physically present on the same site as the robot and communicate with it by using speech and gestures, or he can use telepresence from a distant place and communicate via the Internet. In each case, communication takes place via a new type of user interface based on multimedia and cognition. In this paper we describe the mechatronic structure, the functional subsystems and motion control principles of the robot, as well as discuss the challenges of the development project.