This article describes the functional anatomy of the muscles that contribute to the generation of ventilatory pressures. The physiologic principles that guide their function are analyzed, and their composition and metabolism are described, inasmuch as this affects their adaptation and capacity to face acute and chronic loads. Special emphasis is placed on the clinical and laboratory evaluation of their function. In addition, the concept of respiratory muscle fatigue is developed so the reader can place all these concepts in the context of their clinical applications.