The purpose of this chapter is to provide the reader with a better understanding of the theory and practice of providing effective levels of challenge for people with motor disability, using rehabilitation robotics to provide the safety and assurance that is necessary to prevent physical harm and mental frustration. First, we describe the therapeutic context with which clinicians encounter the need to design challenge into the motor learning sessions that are typical for individuals who are recovering from impaired movement. Second, we explore the challenge point framework as a major breakthrough in our understanding of the nature of challenge in motor performance and how this challenge contributes to efficacious motor learning. Next, we describe ways in which rehabilitation robotics can be designed and implemented to explore the ways in which people with motor disability can learn to move again and how results with these devices suggest extending the challenge point framework to take into account self-efficacy and willingness to practice. Finally, we provide a detailed example of a robotic system that works collaboratively with the clinician to provide physical challenge during walking and balance training in people with poststroke hemiparesis using a library of novel techniques. We conclude by providing further thoughts to engineers and clinicians who collaborate to develop a next generation of rehabilitation robotics that build on the concepts of optimal challenge into the engineering design.