Electrodiagnosis, which includes nerve conduction and needle electromyographic studies, is an essential element in the evaluation of peripheral neuropathies. A systematic approach to the electrodiagnostic evaluation aids in clarifying the distribution and extent of involvement, type of nerve damage, and time course. When these data are combined with clinical information, a full characterization of the neuropathy is possible leading to a sound differential diagnosis and selection of rational tests to order. This article will discuss the relevant anatomy and physiology of peripheral nerves and the principles of nerve conduction and needle electromyographic studies, including how they help distinguish between prototypic examples of primary axonal and demyelinating neuropathies, how to plan electrodiagnostic studies, and how to interpret data from outside studies.