A number of nonheme iron complexes have recently been identified that catalyze the epoxidation and cis-dihydroxylation of olefins with H2O2 as oxidant. These catalysts have been inspired by a class of arene-dihydroxylating enzymes called the Rieske dioxygenases, the active sites of which consist of an iron center ligated by two histidines and a bidentate aspartate residue. The two remaining sites are cis to each other and utilized for oxygen activation. The most effective biomimetic catalysts thus far have polydentate ligands that provide two such cis-oriented labile sites to activate the H,02 oxidant. This overview summarizes recent developments in this sub-field of bio-inspired oxidation catalysis and discusses the evolution of the mechanistic pathways proposed to rationalize the new experimental results and the dichotomy between olefin epoxidation versus cis-dihydroxylation. (C) 2006 Elsevier B.V. All rights reserved.