The impact of early transition-metal oxygen-anion cluster (polyoxometalate or "POM") geometric and electronic structural properties on POM-based catalysis is addressed. Three specific areas of general challenge in POM-based catalysis and catalytic materials are elaborated: (1) the role of ion pairing in catalyst stability, selectivity, and reactivity; (2) the presence of multiple reactive forms of the POMs and the interconvertability of these forms under turnover conditions; and (3) the impact of POM ground state and excited state electronic structure on turnover and selectivity. (c) 2006 Elsevier B.V. All rights reserved.