To enrich legged robots with fast energy-efficient mobility on even terrain, wheeled-legged robots have emerged as a valued robot form in robotics research. This paper describes the complete development of a new wheeled-legged quadruped robot named Max, ranging from its mechanical design over system architecture to core algorithms implemented for it to realize various motion behaviors. Instead of attaching wheels to the distal ends of legs as in the existing wheeled-legged robot designs, this robot has wheels installed on the knees with a special switching mechanism to convert a leg between the legged and wheeled locomotion modes. This design keeps the wheeled leg lightweight, enabling the robot to preserve the motion agility as a quadruped robot while gaining the energy-efficiency as a four-wheel or even two-wheel mobile robot. An online locomotion generation method is proposed to compute the 6-D body trajectory of the robot in walking on the perceived terrain, while dynamic movements such as leaps and flips are generated by a unified trajectory optimizer, which is also used to generate the transition motions of the robot to transform into the wheeled mode. The diverse mobility of the proposed robot Max is verified with extensive experiments. Note to Practitioners—Empowering robots with all-terrain mobility is a fundamental open problem in developing a new generation of robots. To this end, combinations of wheels and legs have been explored for robots to possess both traversability on uneven terrains and efficiency on even terrains. This paper proposes a new wheeled-legged quadruped robot with focuses on the integrated design of wheeled legs, system architecture, and core algorithms implemented for various legged and wheeled locomotion behaviors. To embed wheels without adding additional motors and keep the light weight of original legs, a special switching mechanism is designed and integrated at the knee joints where wheels are installed. Algorithms for generating...