In order to measure the efficacy of a mouthwash against the dental plaque two different concepts should be defined: the substantivity and antiplaque effect. The substantivity of an oral antiseptic is defined as the prolonged adherence to the oral surfaces and its slow release at effective doses which guarantee the persistence of the antimicrobial activity. The more substantivity an oral antiseptic has the better. For its study in vivo, the most popular models are those which analyze the effect that a single mouthwash has in a mature biofilm.