Inspiration during quiet breathing is primarily dependent on contraction of the diaphragm that leads to expansion of the thoracic cage with subsequent pressure changes that draws in air. Expiration, in contrast, is generally passive with relaxation of the diaphragm. During exercise, inspiration and expiration become deeper to meet the body’s metabolic demands, incorporating accessory muscles to further expand and contract the thoracic cavity. Neuromuscular diseases lead to an impairment of the ability to meet this extra demand, depending on which muscles are affected. Inspiratory muscle weakness can progress to hypoventilation.Patients will report dyspnea on exertion. Expiratory weakness may lead to impaired airway clearance and increase the risk of respiratory tract infections. This chapter reviews the pathophysiology of respiratory muscle weakness and discusses noninvasive tests designed to assess inspiratory and expiratory muscle strength that are used clinically to monitor progression of illness and effectiveness of treatment, and to decide when to initiate ventilatory support and airway clearance devices.