The ventilatory muscles are of primary importance in the maintenance of ventilation. This rather complex system of muscles centers around the diaphragm. As diaphragmatic function becomes compromised with the progression of different lung diseases, the participation of other muscles becomes necessary. This is clinically manifested by the recruitment of many of these muscles even during quiet breathing. The use of simple questions during a medical history, determination of the respiratory rate, assessment of the pattern of breathing, and observation of thoracoabdominal movements are helpful in the initial evaluation. Measurement of the FVC, lung volumes, and tidal breathing help direct attention to more specific investigation of the ventilatory muscles. Decreased respiratory muscle strength can be confirmed by measurement of PImax and PEmax. Decreased respiratory muscle endurance can be readily ascertained by measuring the MVV. Use of these simple techniques, available in most laboratories, is appropriate for initial evaluation and establishing a diagnosis. The additional measurements of esophageal and gastric pressures have added a new dimension to the study of the diaphragm; these techniques, however, remain a research tool.