Spontaneous activities are a major semeiologic sign in electromyography. The present article deals with the different aspects recorded in practice in normal and pathological cases. There are two types of spontaneous activities, those related to motor unit hyperactivity (fasciculations and myokymia) and those related to the hyperactivity of one or more muscle fibers: fibrillations, positive sharp waves, myotonic discharges and complex repetitive discharges. In the first case the lesion is located in the axone and in the second in the membrane of the muscle fibers; All theories related to the cells' abnormalities share a common feature: spontaneous activities result from abnormal firing of the membrane action potential of muscular fibers. This functional abnormality may results from different types of lesions within the cells' membrane and determines the aspects of spontaneous activities. Impaired function of muscular cells' membranes can be produced by denervation or lesion of the membrane structure itself. The latter can be multiple and linked with the membrane proteins (such as laminine or dystrophin as in AIDS diseases) or with ion channel disturbances. Multiple membrane cell alterations may produce the same kind of spontaneous activity; for instance, myotonic discharges have the same morphology in Thomsen and Steinert's disease despite their different mechanisms and fibrillations seen in denervations and myopathies. The practical consequences are discussed and a new classification of these spontaneous activities is proposed.