Advances in design of ruthenium catalysts for olefin metathesis are described, in which problems of catalyst decomposition via formation of chloride-bridged species are addressed by replacing the chloride ligands of the Grubbs-class catalysts with aryloxides as "pseudohalide" ligands. The best of the new catalysts offer activity comparable to or greater than that of the parent chloride systems in ring-closing metathesis, with some intriguing differences in selectivity, particularly in macrocycle formation. Unexpected advantages associated with removal of spent catalyst following reaction are also reviewed. (c) 2006 Elsevier B.V. All rights reserved.