The structural and functional organization of the diaphragm muscle is described in terms of the essential units for neuromotor control. These motor units in the diaphragm vary markedly in their metabolic, contractile, and fatigue properties. Thus, the forces generated by the diaphragm during different ventilatory and nonventilatory behaviors will depend on the number and type of motor units recruited. It is therefore important to understand the basic properties of diaphragm motor units and the mechanisms underlying their recruitment.