Many bacteria, fungi and viruses colonise different sites in the healthy mouth. These microbes maintain relationships with the host based on mutual benefits and have a fundamental role in maintaining a normal oral physiological environment. Members of one species gain benefits, while those of the other species neither benefit nor are harmed. These organisms are known as commensal organisms. The word ‘commensalism’ is derived from the word ‘commensal’, meaning ‘eating at the same table’. These live in a symbiotic relationship where one organism lives near, on or within another organism and benefits from it without injuring or helping the other. Not only do these organisms not cause harm, but they may keep pathogenic species in check by not allowing them to adhere to mucosal surfaces.