In many cases, one encounters great difficulties in finding causes of polyneuropathies among some 100 etiologies. However, in current practice, the diagnostic span is not so large. At this point, it is usual to get some aid from electrophysiologic tests in order to determine the nature of the peripheral nerve disorder. Examination with needle electrodes is mainly useful to detect fibrillation potentials or positive sharp waves which are indicative of an acute or subacute axonal neuropathy in which the process of degeneration is more important than the capacities of regeneration. Studies of sensory and motor nerve conduction velocities is of a more important utility to separate the different types of peripheral neuropathies: axonopathies in which nerve conduction velocities are normal or slightly decreased but in which muscular evoked potentials and sensory potentials are reduced, myelin disorders in which nerve conduction velocities are markedly decreased and in which finding of conduction blocks allows to individualize two forms, motor and sensory neuronopathies, where the pure lesion of the motor or sensory cell bodies correlate with the normality of sensory and motor pathways respectively.