The conduction block (CB) of the nerve impulses in the peripheral nervous system induces a partial or complete loss of the nerve function, usually more marked on the motor fibers. The CB is due to a focal loss of the electrical properties of the nerve fibers. The underlying mechanisms include paranodal and segmental demyelination, and physical or chemical dysfunction of the ionic channels. The electrophysiological study allows distinction of axonal lesion from dysfunction due to CB. Recognition and quantification of the CB requires criteria that are not yet widely accepted. The different modalities of the examination are detailed and the criteria are discussed. The peripheral neuropathies with CB have different aetiologies; physical (electrical, thermal, percussion, compression or constriction injuries, radiation-induced), toxic (acting either on the axon or the myelin), more rarely ischaemic (arteriopathy or vasculitis), but frequently inflammatory (acute or chronic, primary or secondary) or related to dysimmune states (multifocal sensory-motor and motor neuropathies with persistent CB). Their review allows a discussion of their differential diagnosis in relation to the underlying mechanisms, site and duration (transient to persistent) of the CB.