Generally, mesocrystals are known to be biological and bio-inspired mesoscale structures consisting of oriented nanometric crystalline units. In this article, mesocrystals are broadly categorized in a new family of crystalline materials as intermediate states between single crystals and polycrystals regardless of their unit size and degree of crystalline order. Here, the formation routes and the specific properties of a variety of mesocrystals and their related structures are described. Moreover, the potential of the novel crystalline materials having a high crystallinity and a high specific surface area is presented on the basis of their functions.