Neuronal and/or axonal hyperactivity and hyperexcitability is an important feature of motor neuron diseases. It results clinically in cramps and fasciculations. It is not specific to motor neuron diseases, and can occur in healthy subjects, as well as in various pathologies of the peripheral nervous system, including nerve hyperexcitability syndromes. Hyperexcitability plays an important and debated role in the pathophysiology of motor neuron diseases, especially in amyotrophic lateral sclerosis (ALS). The mechanisms causing hyperexcitability are not yet clearly identified. While most studies favor a distal axonal origin site of fasciculations, some of the fasciculations could be of cortical origin. The consequences of hyperexcitability are also discussed, whether it is rather protective or deleterious in the disease course. Fasciculations are depicted both clinically and using electromyogram, and more recently the interest of ultrasound has been highlighted. The importance of fasciculation potentials in the diagnosis of ALS led to changes in electrophysiological criteria at Awaji consensus conference. The contribution of these modifications to ALS diagnosis has been the subject of several studies. In clinical practice, it is necessary to distinguish fasciculations potentials of motor neuron disease from benign fasciculations. In most studies of fasciculation potentials in ALS, the presence of complex fasciculation potentials appears to be relevant for the diagnosis and the prognosis of the disease.