Critical illness polyneuropathy (CIP) along with critical illness myopathy (CIM) is a frequent condition in critically ill patients. Similar to critical illness myopathy, the exact incidence, etiology, and mechanisms of CIP are not well understood. Although decreased compound muscle action potential amplitudes may be seen in both conditions, differentiation mostly relies on sensory nerve conduction studies which require special care to perform at intensive care units. In some instances, both CIP and critical illness myopathy are observed, and this condition is called critical illness neuromyopathy. In this review, nerve conduction studies and needle electromyography for the diagnosis of CIP and differentiation of CIP from critical illness myopathy and other conditions at intensive care units are summarized.