Primary muscle cramps, without known cause, are very frequent especially in the elderly and during the night. They are different from secondary cramps. Likewise they are to be separated from several syndromes erroneously quoted as cramps. The pathophysiological mechanism seems due to result from an initial dysfunction in the distal part of the motoneuron. When the cramps are severe, differential diagnosis with amyotrophic lateral sclerosis may be difficult. Quinine is the best empiric treatment largely used in spite of moderate side effects.