Molecular metal oxides, so-called polyoxometalates (POMs), are an emerging class of inorganic materials. Their well-defined molecular structure and tuneable properties and reactivity have made POMs ideal analogues for the corresponding solid-state metal oxides. This has led to applications in areas including energy conversion and storage, molecular magnetism, and catalysis. Typically, reactivity control in POMs requires their functionalization with reactive metal centres: In molecular magnetism, the incorporation of lanthanides in POMs is a widely used principle, while energy conversion and storage schemes often use transition metal functionalized POMs, In catalysis, the introduction of lews-acidic sites in POMs is a well-known concept. Thus, robust and broad-scope synthetic concepts for the metal functionalization of POMs are critical for the targeted development of new functional compounds. For polyoxotungstates (and, to a lesser degree, for polyoxomolybdates), controlled metal functionalization is possible using so-called lacunary cluster derivatives where one or several of the original metal ions (W or Mo) have been hydrolytically removed from the cluster shell. Selective binding of suitable metal ions at the resulting vacancy then leads to the targeted metal-functionalized species. This approach has led to ground-breaking molecular components, e.g., for catalysis molecular electronics and medicine. In contrast, in polyoxovanadate (POV) chemistry, controlled and predictable metal functionalization approaches are still in their infancy, and the field is currently dominated by a combination of empirical knowledge and serendipity. Thus, the development of controlled approaches for the predetermined and selective metal-functionalization of POMs are critical to enable knowledge-based materials development.