Mesocrystals are inorganic superstructures formed through highly parallel crystallization and controlled alignment of nanoparticles. They represent a new type of colloidal crystal, formed from nonspherical, crystalline building units, and differ from classical crystallization, which relies on ion-by-ion attachment. Mesocrystals are kinetically metastable species or intermediates in crystallization reactions, often observed as single crystals with defects and inclusions. They exhibit nonclassical crystallization, where nanobuilding blocks self-assemble into ordered superstructures, independent of ion products or molecular solubility. This process occurs without changes in pH or osmotic pressure and offers new strategies for crystal morphogenesis. Mesocrystals have been observed in various systems, including biominerals and synthetic inorganic materials. For example, in biomineralization, organic additives can be incorporated into the crystal structure, a phenomenon not explained by classical crystallization models. Similarly, synthetic mesocrystals have been formed with high orientational order, sometimes containing organic additives as defects. These structures are often more symmetric than their constituent nanoparticles, suggesting multiple twinning of primitive units. Mesocrystals can be one- or two-dimensional, formed by the alignment of nanoparticles, or three-dimensional, formed through oriented attachment or self-assembly. Examples include hexagonal prismatic fluoroapatite seed crystals, calcite mesocrystals, and hematite mesocrystals. These structures exhibit high orientational order and can be influenced by factors such as polymer additives, solubility, and ionic strength. Mesocrystal formation is a complex process involving nanoparticle aggregation, self-assembly, and controlled alignment. It can occur in gels, solutions, or under specific conditions of supersaturation. The formation of mesocrystals is often a kinetic, metastable intermediate rather than a thermodynamically stable product. The mechanisms of mesocrystal formation involve interactions such as dipole fields, anisotropic van der Waals forces, and selective adsorption of additives onto specific crystal faces. Mesocrystals have significant implications for materials science, offering new possibilities for the synthesis of crystalline nano and mesostructures. They challenge classical crystallization models and suggest that mesocrystals may be common intermediates in many crystallization reactions, particularly under conditions of high supersaturation or low solubility. The study of mesocrystals provides insights into the mechanisms of self-assembly and the formation of ordered superstructures, with potential applications in nanotechnology, biomineralization, and materials engineering.Mesocrystals are inorganic superstructures formed through highly parallel crystallization and controlled alignment of nanoparticles. They represent a new type of colloidal crystal, formed from nonspherical, crystalline building units, and differ from classical crystallization, which relies on ion-by-ion attachment. Mesocrystals are kinetically metastable species or intermediates in crystallization reactions, often observed as single crystals with defects and inclusions. They exhibit nonclassical crystallization, where nanobuilding blocks self-assemble into ordered superstructures, independent of ion products or molecular solubility. This process occurs without changes in pH or osmotic pressure and offers new strategies for crystal morphogenesis. Mesocrystals have been observed in various systems, including biominerals and synthetic inorganic materials. For example, in biomineralization, organic additives can be incorporated into the crystal structure, a phenomenon not explained by classical crystallization models. Similarly, synthetic mesocrystals have been formed with high orientational order, sometimes containing organic additives as defects. These structures are often more symmetric than their constituent nanoparticles, suggesting multiple twinning of primitive units. Mesocrystals can be one- or two-dimensional, formed by the alignment of nanoparticles, or three-dimensional, formed through oriented attachment or self-assembly. Examples include hexagonal prismatic fluoroapatite seed crystals, calcite mesocrystals, and hematite mesocrystals. These structures exhibit high orientational order and can be influenced by factors such as polymer additives, solubility, and ionic strength. Mesocrystal formation is a complex process involving nanoparticle aggregation, self-assembly, and controlled alignment. It can occur in gels, solutions, or under specific conditions of supersaturation. The formation of mesocrystals is often a kinetic, metastable intermediate rather than a thermodynamically stable product. The mechanisms of mesocrystal formation involve interactions such as dipole fields, anisotropic van der Waals forces, and selective adsorption of additives onto specific crystal faces. Mesocrystals have significant implications for materials science, offering new possibilities for the synthesis of crystalline nano and mesostructures. They challenge classical crystallization models and suggest that mesocrystals may be common intermediates in many crystallization reactions, particularly under conditions of high supersaturation or low solubility. The study of mesocrystals provides insights into the mechanisms of self-assembly and the formation of ordered superstructures, with potential applications in nanotechnology, biomineralization, and materials engineering.