Bruxism is a para-function with tooth clenching and grinding. Particularly, the sleep bruxism is a frequent phenomenon that causes cephalalgia and muscular/joint pains to the awakening, besides an increased dental abrasion and sensibility. The etiology of sleep bruxism is uncertain: while the occlusal discrepancies and the anatomy of the bony structures of the orofacial region play only a minor role, others factors, like smoking, alcohol, drugs, systemic diseases, stress, trauma and heredity, appear to have an important role in the sleep bruxism genesis. Recent polysomnographic studies, suggest that sleep bruxism episodes are part of an sleep arousal response. The sleep arousal response is a sudden change in the depth of sleep. Besides the sleep bruxism appears to be an disturbance in the dopaminergic system. Further psychological factors (like stress, anxiety) are implicated in the aetiology of sleep bruxism as well. The aim of this paper is to review the literature on the aetiology of bruxism.