This paper presents a short review related to the chemistry of the Ru=O group with data extracted from recent reports described in the literature by the authors of the present review and others. It was presented as an oral contribution to the 2005-Adhoc meeting held in Koln, Germany. The main thermodynamic characteristics of mononuclear complexes containing the Ru=O group are described and the effects produced by different type of electronic perturbations are analyzed through the ligands. Furthermore, an example of heterogenization of a Ru=O complex through a polypyrrol conductive polymer on a graphite electrode and its consequences are also described. Finally a description of complexes bearing two Ru=O groups is carried out with special attention to those complexes that in their higher oxidation states are capable of oxidizing water to molecular dioxygen. (c) 2006 Elsevier B.V. All rights reserved.