Single atom catalysts—SACs—have received intense interest in sustainable energy research due to their enormous application potential and broad catalytic scope. In particular, SACs anchored on molecular metal oxides—polyoxometalates (POMs)—offer unrivalled possibilities as models to understand the function of these complex systems on the atomic level. Research questions which are difficult to address for classical heterogeneous SACs can be addressed by experiment and theory using POM-SACs as prototype catalysts. This review reviews the emerging field of POM-SAC research with a focus on fundamental properties and their application in energy conversion and storage technologies including water splitting, CO/CO2- as well as N2-activation. Current limitations of the field are identified and possible routes to overcome these are described. Future areas of development, including polyoxometalate-single-site catalysts, are highlighted, and critically assessed to enable the community to develop this fast-moving field further.