An authoritative up-to-date summary of the chemistry and applications of polyoxometalates with emphasis on new synthetic strategies directed towards functionalized organic derivatives, self-assembly of mesoscopic composite polyoxoanions, generation of framework materials and thin oxide films, extended optical, magnetic, and electrical properties, applications in homogeneous and nanocluster-based catalysts, phoatalytic water decontamination, ribosomal crystallography, and topological aspects of large symmetrical structures.