Myopathies are rare diseases. They may be genetic (muscular dystrophies, metabolic or congenital myopathies) or acquired (inflammatory, drug-related or toxic myopathies and those due to systemic disease). Muscular abnormalities secondary to affections of the peripheral nervous system or anterior horn are not strictly speaking myopathies. Morphological examination of muscle tissue is the key stage of the diagnostic workup, and crucial in directing patient care. Muscle biopsy analysis takes into account clinical and paraclinical data and requires close collaboration between the clinician and pathologist. Three illustrative examples are presented: a congenital muscular dystrophy, a glycogenosis and a muscular dystrophy mimicking polymyositis.