An order fulfillment system is disclosed including a storage structure and mobile robots capable of moving vertically and horizontally within the storage structure to transfer product totes to and from storage locations within the storage structure. The mobile robots may move along a track system including horizontal rails for horizontal travel of the mobile robot and vertical towers for vertical travel of the mobile robot. The vertical towers include two opposed pairs of tracks having U-shaped slots. The order fulfillment system may further comprise a mobile robot configured to travel vertically and horizontally along the track system to transfer product containers to and from the storage locations in the storage structure, and horizontally about a horizontal deck spanning the storage aisles. The mobile robot includes four wheel assemblies, each assembly including a pair of wheels mounted on ends of a climbing link. When positioned in a vertical tower, the climbing links rotate so that the drive wheels climb up or down the storage structure by engaging within the U-shaped slots of the track. The same wheels may be rotated to facilitate horizontal motion.