Acute and subacute polyneuropathies present diagnostic challenges since many require prompt initiation of treatment in order to limit axonal degeneration and since an exact and detailed diagnosis is a prerequisite for making the correct choice of treatment. It is for instance of utmost importance to recognize whether the underlying pathological changes are due to demyelination or to axonal degeneration and electrodiagnostic tests can thus in most cases contribute considerably to the securing of an exact diagnosis. The specific and characteristic electrophysiological findings in the different types of acute and subacute neuropathies are discussed, and the various electrophysiological pitfalls and technical problems, which are met in these patients, are mentioned.