Respiratory muscle dysfunction encompasses a wide variety of conditions, which may present with a wide variety of symptoms. This chapter builds up from a brief description of respiratory muscle physiology to respiratory muscle pathology. It incorporates a systematic approach to respiratory muscle dysfunction, beginning with a syndrome-based approach that enables the differentiation of the various causes of respiratory muscle dysfunction from one another. Physiological, imageological and other tools for workup are discussed. As well, the chapter explores the complications of respiratory muscle dysfunction that must be sought in all cases and discusses how these may vary both by symptomatology and degree of severity, based on the specific muscles involved.