Two distinct areas of development contribute to the importance of enzymology in skeletal muscle disease: the measurement of enzymes in serum as a diagnostic aid and the investigation of enzyme deficiencies as the cause of metabolic myopathy. Currently, serum enzyme measurement, particularly creatine kinase, is used to aid in the detection of suspected myopathy, to differentiate myopathy from neurogenic disease, to identify dystrophies at a preclinical stage, to detect female carriers of dystrophies, and to assess response to therapy. Metabolic myopathies are disorders of muscle energy production that result in skeletal muscle dysfunction. The biochemical basis of such disorders is often a specific single enzyme defect. The tremendous advances in the molecular genetics of such disorders has added remarkably to our understanding of the primary defects involved and possible heterogeneity displayed at the molecular level.