Mesocrystals (MCs) are superstructures with a crystallographically ordered alignment of nanoparticles. Owing to their organized structures, MCs posses some unique characteristics such as a high surface area, pore accessibility, and good electronic conductivity and thermal stability; thus, MCs could be beneficial for many areas of research and application. This review begins with a description of the common synthesis strategies for, and characterization and fundamental properties of metal oxide MCs. Newly developed analytical methods (that is, photoconductive atomic force microscopy and single-molecule, single-particle fluorescence microscopy) for unraveling the charge transport and photocatalytic properties of individual MCs are then introduced. Further, recent developments in the applications of various metal oxide MCs, especially in the fields of energy conversion and storage, are also reviewed. Finally, several perspectives in terms of future research on MCs are highlighted. Mesocrystals are superstructures with a crystallographically ordered alignment of nanoparticles and posses unique characteristics such as a high surface area, pore accessibility, and good electronic conductivity and thermal stability. This review summarizes the recent developments of metal oxide mesocrystals in the fields of energy conversion and storage. Mesocrystals are periodic arrangements of nanoparticles that form larger structures of several hundreds of nanometers, or even micrometers, in size. Takashi Tachikawa and Tetsuro Majima from Osaka University in Japan review the potential of metal oxide mesocrystals for applications in energy generation and energy storage. Nanoparticles can have very different properties compared to the regular bulk material from which they originate. For example, the larger surface area of mesocrystals is more efficient for water splitting or catalysis. Additionally, the nanoparticles in mesocrystals are held in place, which can further improve their properties — such as the ability to transport electrical charges. Mesocrystals containing titanium dioxide nanoparticles are of particular interest for use in catalytic processes or as electrodes in lithium batteries. Nevertheless, the assembly of these nanoparticle superstructures, which can require complex fabrication processes, has hampered technological applications so far.