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Introduction: Molar incisor hypomineralization (MIH) and hypomineralized second primary molars (HSPM) are qualitative and asymmetric enamel defects. MIH affects at least one permanent first molar and can also be associated with permanent incisors. HSPM affects at least one primary second molar and possibly primary canines. Hypomineralized enamel displays specific characteristics: the enamel prisms are disorganized, less distinct, the interprismatic space is more marked, the mineral density is decreased, and the protein content is increased. Currently, etiologies remain unknown, but the various studies tend toward a multifactorial model with several systemic, genetic, and/or epigenetic factors, acting in a synergistic or additive way. Materials and method: The authors highlight the various factors involved in diagnosing MIH and HSPM. A review of the prevalence (in France and worldwide) and etiologies of these pathologies is also provided, to enable practitioners to answer any questions parents may have. Conclusion: Having knowledge of these different elements in terms of diagnosis, structure, prevalence, and etiologies will allow the orthodontist to better collaborate with the dentist, but also with the parents in order to ensure adequate dental and orthodontic management.