In this chapter, the authors use the computation, anatomy, and physiology (CAP) principles to consider the impact of common clinical problems on action. They focus on 3 major syndromes: paresis, apraxia, and ataxia. They also review mechanisms that could account for spontaneous recovery, using what is known about the best-studied clinical dysfunction—paresis—and also ataxia. Together, this and the previous chapter lay the groundwork for the third chapter in this series, which reviews the relevant rehabilitative interventions.