Neuromuscular disorders comprise a diverse group of human inborn diseases that arise from defects in the structure and/or function of the muscle tissue — encompassing the muscle cells (myofibres) themselves and their extracellular matrix — or muscle fibre innervation. Since the identification in 1987 of the first genetic lesion associated with a neuromuscular disorder — mutations in dystrophin as an underlying cause of Duchenne muscular dystrophy — the field has made tremendous progress in understanding the genetic basis of these diseases, with pathogenic variants in more than 500 genes now identified as underlying causes of neuromuscular disorders. The subset of neuromuscular disorders that affect skeletal muscle are referred to as myopathies or muscular dystrophies, and are due to variants in genes encoding muscle proteins. Many of these proteins provide structural stability to the myofibres or function in regulating sarcolemmal integrity, whereas others are involved in protein turnover, intracellular trafficking, calcium handling and electrical excitability — processes that ensure myofibre resistance to stress and their primary activity in muscle contraction. In this Review, we discuss how defects in muscle proteins give rise to muscle dysfunction, and ultimately to disease, with a focus on pathologies that are most common, best understood and that provide the most insight into muscle biology.