The main investigations routinely used for the diagnosis of muscle disease include biological and electrophysiological analyses, as well as muscle biopsy. The results of all these tests are discussed in order to their diagnostic and prognostic value in suspected or known muscle disease. However, the diagnosis can only be confirmed when all the data has been obtained: clinical, biological, electrophysiological, biopsy as well as familial or genetic data. When the data are discordant, it should be possible to proceed to nuclear magnetic resonance spectroscopy, and mitochondrial enzyme profiles.