Motor neuron disorders (MND) form a heterogeneous group of neurodegenerative affections: phenotypic description is based on selective injury to the upper motor neuron or lower motor neuron or both. Phenotypic heterogeneity is also present concerning genetic features: genetic factors involved in MND may be causative or susceptibility factors. Consequences of genetic abnormalities lead to metabolic or functional cellular disturbances that are apparently specific for motor neuron disorder. Genetics greatly contribute to our understanding of the pathophysiological mechanisms of motor neuron degeneration. Genetic studies provide pathological hypotheses considering the function of protein encoded. Moreover, when a gene mutation is identified, animal models can be developed to search for modifications induced by the mutation. We propose to detail causative and susceptibility genetic factors involved in MND and to discuss pathological mechanisms that may explain motor neuron death.