Peroneal neuropathy is one of the common focal mononeuropathies in the lower extremities occurring in both adults and children. Foot drop due to weakness of ankle dorsiflexion is the most common presentation of a peroneal neuropathy. It may also result from other causes involving the upper or lower motor neurons. Disorders that must be distinguished from peroneal neuropathy include sciatic mononeuropathy, lumbosacral plexopathy, motor neuron disease, polyneuropathy, and an L5 radiculopathy. To establish a diagnosis, electrodiagnostic studies have been used to localize the level of the abnormality and to establish prognosis. The most common site of injury is the fibular head, but focal neuropathies have also been reported at the level of the calf, ankle, and foot. In this article, we overviewed the peroneal nerve palsy, and its diagnosis by neurophysiologic evaluation, conduction study and needle EMG. The neurophysiologic information gives us the underlying pathophysiology and its prognosis. Therefore the neurophysiologic evaluation must be performed not only for the differential diagnosis, but also for planning the treatment strategy.