Unique structures and functions of porous ionic crystals (PICs) based on polyoxometalate (POM) anions, which are different from typical porous crystalline materials such as zeolites and metal-organic frameworks (MOFs), can be summarized as follows. (i) Crystal structures of PICs are often flexible since Coulomb interaction works isotropically in a long-range, so that structural transformation occurs to adapt to specific guest molecules. (ii) POMs show reversible redox properties leading to the formation of "redox-active" porous materials. (iii) Ionic building blocks create strong electrostatic fields in the pore, which are suitable for accommodating and stabilizing polar guests and ionic intermediates. (iv) Various cations can be incorporated as counter cations of POMs to fine-tune the functions. The crystal structures and functions of PICs can be controlled by the appropriate choice of molecular cations (macrocations) as ionic building blocks as well as the symmetry, size, and charge of POMs. In particular, oxo-centered trinuclear metal (M-III) carboxylates with a general formula of [M3O(OOCR)(6)(L)(3)](+) as macrocations allow controlling the crystal structures of PICs through the bridging ligands (R) and terminal ligands (L). The fine-tuned structures of PICs lead to unique functions in gas/solvent sorption/separation, heterogeneous catalysis, redox-induced ion-uptake/release, synthesis of mixed-valance metal nanoclusters, and so on.