Assessment of inspiratory and expiratory pressure development is a standard test used to evaluate patients with breathing disorders. The purpose of the present article is to examine the utility and limitations of these measurements. Measurement of maximum inspiratory pressure (Pimax) and maximum expiratory pressure (PEmax) generation requires, first and foremost, a cooperative patient and adequate instruction in performance of the required maneuvers (i.e., a maximal inspiratory effort performed against an occluded airway for determination of Pimax and a maximal expiratory effort against an occluded airway for measurement of PEmax). To adequately interpret Pimax and PEmax measurements, one must be aware of the physiologic variables that influence these pressure measurements, including lung volume and the static recoil pressure of the respiratory system. In addition, because these parameters are effort dependent, reproducibility is a potential problem. It is also important to realize that Pimax and PEmax measure only maximal respiratory force generation and are not indicators of “low frequency” muscle force-generating capacity or muscle endurance. Nevertheless, assessment of Pimax and PEmax is important in several clinical conditions including (1) defining the differential diagnosis in patients manifesting a restrictive ventilatory defect during routine pulmonary function testing, (2) determining the severity of respiratory muscle involvement in patients with a known myopathy, (3) assessing the need for ventilatory support in myopathic disorders, (4) evaluating patients with acute respiratory failure of unknown origin, (5) providing information about respiratory muscle dysfunction in patients with systemic disturbances (e.g., sepsis) that affect muscle performance, (6) providing prognostic information in patients with known disorders that affect muscle function, and (7) evaluating patients to be weaned from mechanical ventilation.