Atomically precise clusters are valuable building blocks for material science. Clusters are used to build stimuli-responsive materials, or hierarchical (porous) structures. Clusters can be precursors or intermediates in nanocrystal synthesis. Composites with improved strength are produced by incorporating zirconium oxo clusters in polymers. Zirconium and hafnium oxo clusters are used as catalysts to make or break amide bonds, and they are are suitable as inorganic extreme-ultraviolet photoresists. Subtle differences in cluster chemistry between Zr and Hf are exploited in the industrial separation of Zr from Hf. The whole field of group 4 MOFs is based on group 4 metal oxo clusters that act as secondary building units (or nodes) in the MOF framework. Apart from the typical Ti8, Zr6, and Hf6 oxo clusters used in MOFs, many discrete group 4 oxo clusters exist with different nuclearities. Discrete group 4 oxo clusters are conceptually close to colloidal oxide nanocrystals and share some features with them. For example, both have an inorganic core, usually capped by an organic ligand shell. Discrete oxo clusters are usually synthesized with short and rigid ligands. The resulting clusters crystallize from the reaction mixture, allowing for convenient purification and structural characterization by single crystal X-ray diffraction (XRD). For example, Zr6-carboxylate clusters have the general formula: Zr 6 O 4 (OH) 4 (OOCR) 12 .