&NA;This paper reviews the recent advances in our knowledge of muscle disease. The use of muscle biopsy for diagnosis is discussed. The etiology, pathogenesis, and treatment of polymyositis/dermatomyositis are considered. The author discusses the clinical patterns, inheritance, and pathogenesis of progressive muscular dystrophies, especially Duchenne muscular dystrophy; myotonic disorders; glycogen storage diseases; disorders of lipid metabolism; mitochondrial diseases; and congenital muscle diseases. (Neurosurgery,5: 747‐758, 1979)