The article discusses the formation and properties of mesocrystals, which are inorganic superstructures formed through highly parallel crystallization and controlled alignment of nanoparticles. Mesocrystals are observed as kinetically metastable species or intermediates in crystallization reactions, leading to single crystals with defects and inclusions. The formation of mesocrystals involves a modular nanobuilding-block route, which is distinct from classical crystallization that proceeds through ion-by-ion attachment. This approach offers new strategies for crystal morphogenesis, independent of ion products or molecular solubility, and without changes in pH or osmotic pressure. The review highlights the importance of mesocrystals in modern colloid and materials chemistry, drawing parallels with biomineralization processes. Biominerals, such as those found in sea urchins and coccoliths, exhibit complex morphologies and hierarchical order, often containing organic additives. Mesocrystals share similar characteristics, with high orientational order and the ability to form superstructures from nanoparticles. The article also reviews early reports on mesocrystals, including studies on CeIV compounds, CuO, and CaCO3, where mesocrystal intermediates were observed. Later, the formation mechanisms of one- and two-dimensional mesocrystals are discussed, followed by detailed studies on three-dimensional mesocrystals formed in gels and solutions. Examples include fluorapatite, hematite, and copper oxalate, where the role of additives and functional groups in controlling mesocrystal morphology is explored. The review concludes by discussing the implications of mesocrystal formation for crystallization processes, suggesting that mesocrystals may be regular intermediates in ordinary crystallization reactions, especially in systems with low molecular solubilities or high supersaturations. The formation of mesocrystals challenges traditional crystallization theories and opens new avenues for the design of crystalline nano and mesostructures.The article discusses the formation and properties of mesocrystals, which are inorganic superstructures formed through highly parallel crystallization and controlled alignment of nanoparticles. Mesocrystals are observed as kinetically metastable species or intermediates in crystallization reactions, leading to single crystals with defects and inclusions. The formation of mesocrystals involves a modular nanobuilding-block route, which is distinct from classical crystallization that proceeds through ion-by-ion attachment. This approach offers new strategies for crystal morphogenesis, independent of ion products or molecular solubility, and without changes in pH or osmotic pressure. The review highlights the importance of mesocrystals in modern colloid and materials chemistry, drawing parallels with biomineralization processes. Biominerals, such as those found in sea urchins and coccoliths, exhibit complex morphologies and hierarchical order, often containing organic additives. Mesocrystals share similar characteristics, with high orientational order and the ability to form superstructures from nanoparticles. The article also reviews early reports on mesocrystals, including studies on CeIV compounds, CuO, and CaCO3, where mesocrystal intermediates were observed. Later, the formation mechanisms of one- and two-dimensional mesocrystals are discussed, followed by detailed studies on three-dimensional mesocrystals formed in gels and solutions. Examples include fluorapatite, hematite, and copper oxalate, where the role of additives and functional groups in controlling mesocrystal morphology is explored. The review concludes by discussing the implications of mesocrystal formation for crystallization processes, suggesting that mesocrystals may be regular intermediates in ordinary crystallization reactions, especially in systems with low molecular solubilities or high supersaturations. The formation of mesocrystals challenges traditional crystallization theories and opens new avenues for the design of crystalline nano and mesostructures.