Critical illness myopathy (CIM) is a primary myopathy associated with increased mortality and morbidity, which frequently develops in severely ill patients. Several risk factors have been suggested for the development of critical illness myopathy. However, neither the exact etiology nor the underlying mechanisms are known in detail. Although for definite diagnosis muscle biopsy is needed, electrophysiological tests are crucial for the diagnosis of probable critical illness myopathy and differential diagnosis. In this review, conventional electrophysiological tests such as nerve conduction studies, needle electromyography, direct muscle stimulation, and repetitive stimulation for diagnosis of critical illness myopathy are summarized. Moreover, studies using the novel method of recording muscle velocity recovery cycles are addressed.