Disorders of the respiratory muscles (RM) are common, yet are often overlooked. Physical examination findings may suggest the presence of RM weakness but are nonspecific. Several tests of RM function are available, including measurements of upright and supine vital capacity, inspiratory and expiratory muscle pressures, sniff nasal inspiratory pressure, sniff esophageal and transdiaphragmatic pressures, and peak cough flow. These tests are widely available but are effort dependent. Electrodiagnostic studies, such as electromyography, electrical, or magnetic phrenic nerve stimulation, are effort independent but require more specialized equipment and staff. Imaging techniques such as ultrasound provide helpful complementary information about diaphragm function.