The physiological and anatomical properties of mammalian motor units are discussed, and the results of human and animal studies are compared. A physiological organization of motor units based on the mechanical properties of their associated muscle units is examined. It is concluded that such an organizing principle has broad universal application in both animal and human studies. An anatomical organization of muscle units by their association with their physiological properties and the histochemical profile of their muscle fibers is also considered. The anatomical organizing principle also has broad-ranging applicability. The organization of muscle units according to muscle architecture and innervation patterns is described and its potential applicability to considerations of muscle structure and function is discussed. It is concluded that a number of gaps in our knowledge of muscle unit organization have been identified, especially among human studies, but the potential to fill these gaps rapidly is great.