The embedding of molecular metal oxides, or polyoxometalates (POMs), in metal- organic frameworks (MOFs) opens new research avenues in catalysis and beyond. This Review explores the host-guest chemistry of POMs embedded in MOF hosts, and discusses the synergism of the resulting composites for heterogeneous catalysis. The Review focuses on well-established prototype POMs such as Keggin- and Dawson-anion classes, and well-researched MOFs including the MIL, UiO and NENU families. Outstanding examples of synergistic catalytic activity between POM and MOF are described, and key performance parameters including POM localization, pore-size and pore-structure as well as particle size effects are described for technologically important catalytic processes. In addition to thermal catalysis, we discuss the use of POM@MOF composites for electro- and photo-catalysis with an emphasis on energy conversion systems. Finally we provide an outlook at emerging areas, where POM@MOF composites could lead to new catalytic reactivity.

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