Disorders of nerve and muscle are frequent causes of respiratory insufficiency, but current methods often fail to adequately assess the problem. Phrenic nerve conduction and needle electromyography of the diaphragm are of great assistance in identifying the nature and site of the disorder: the various disturbances of central drive, axonal or demyelinating neuropathies of the phrenic nerves, and certain myopathies. These studies may be performed on adults, children, or infants, and in outpatient, general ward, or critical care settings. It is hoped these techniques will stimulate further developments in the field.