Recent developments of ruthenium-allenylidene complexes as precatalysts for alkene metathesis reactions are described. Since the first evidence, reported in 1998 by the Dixneuf and Furstner's groups, a variety of ionic 18-electron and neutral 16-electron ruthenium-allenylidene complexes mainly of the type [RuCl(=C=C=CR2)(arene)(PR3)] [X] and RuCl2 (=C=C=CR2)(L')(L") have been described. The parallel formation and catalytic activity of related complexes in which the allenylidene moiety is formally transformed into an indenylidene ligand, that may take place via intramolecular rearrangement, is discussed. Applications are described for the production of both fine chemicals and polymers. Attempts for catalyst recycling in ionic liquids, or via heterogenization are also presented. (C) 2003 Elsevier B.V. All rights reserved.