Ultrastructural examination of a peripheral nerve biopsy may be particularly useful and sometimes indispensable for identification of the type of nerve lesion and of the aetiologies of peripheral neuropathies. The ultrastructural findings have, anyway, to be correlated with the clinical findings, the electrophysiological examination and the laboratory investigations. In this presentation, the various causes of peripheral neuropathies for which nerve biopsy study by electron microscope can provide diagnostic information are discussed. The principal aetiologies that will benefit from such an ultrastructural study are toxic, infectious, haemopathic and storage disorders. Chiefly for Charcot-Marie-Tooth sporadic cases, there are still indications for nerve biopsy to orientate diagnostic research in molecular biology. Sometimes, the electron microscopic examination will help to determine not only the cause of the peripheral neuropathy, but also the mechanism of nerve lesions which may induce specific and efficient treatments.