Molar incisor hypomineralisation is described as enamel hypomineralisation of systemic origin involving the first permanent molars. It is also often associated with damage to permanent incisors, which undoubtedly have an important function during developmental age. It is estimated that permanent incisors are involved in approximately 30% of patients with molar incisor hypomineralisation. Early diagnosis of molar incisor hypomineralisation, implementation of appropriate treatment and knowledge of the factors that may contribute to this disorder can reduce the risk of loss of the affected teeth. The aetiology of the disorder has not been fully established. Many studies have shown correlations between molar incisor hypomineralisation and a particular aetiological factor. Research is underway to narrow down this area of consideration, as the aetiology identified to date is very extensive and multifactorial. Both genetic, epigenetic and environmental factors influence the onset of molar incisor hypomineralisation. In the large latter group, a particular role in the aetiology of the disorder is attributed to maternal viral infections during pregnancy, as well as chronic maternal diseases such as hypertension or diabetes, maternal use of certain medications during pregnancy, perinatal complications and diseases of early childhood. Many studies indicate that genetic factors and endocrine disturbances are the most important predisposing factors for molar incisor hypomineralisation. This paper discusses the diagnostic challenges and the most likely aetiological factors of molar incisor hypomineralisation investigated to date.