Metabolic myopathies are caused by defects in the metabolic processes of energy storage and utilization, and can present with exercise intolerance, fatigue, muscle pain, and weakness. Metabolic myopathies are rare and can be difficult to diagnose. However, the clinical presentation can be similar to, and thus mimic, both the idiopathic inflammatory myopathies and other genetic muscle disorders including the muscular dystrophies. Careful enquiry about the nature and timing of muscle pain, as well as identification of other clinical ‘red-flags’, can highlight the possibility of a metabolic myopathy. The possibility of metabolic myopathy or muscular dystrophy mimicking myositis should be considered early in ‘treatment-resistant myositis’ or ‘seronegative myositis’. The diagnosis of metabolic myopathies depends on a multidisciplinary team, an awareness of the increasing availability of enzyme activity testing and the utility of expanding genetic technologies. In some cases, dietary manipulation and enzyme replacement therapies are useful treatments.