Molar Incisor Hypomineralisation (MIH) is defined as a hypomineralisation of systemic origin of one to four permanent first molars frequently associated with affected incisors. MIH molars are fragile and caries can develop very easily in those molars. Although MIH molars are well known by paediatric dentists and their occurrence is related in severe cases to major clinical problems, only limited data of the size of the problem are available. The prevalence of MIH ranges in the literature from about 3.6 to 25% and seems to differ in certain regions and birth cohorts. Unfortunately more complete comparable valid data are lacking at the moment. It seems that several aetiological factors can cause the enamel defects and that their occurrence is child related.For children with repeated illnesses in the first years after birth and children with opacities on erupted molars or incisors it seems useful to increase the frequency of dental check-ups during the period of erupting first permanent molars.