Recent advances in olefin metathesis studies include the preparation of catalysts "in situ," supported catalysts, and tungsten-based catalysts, and observation of molybdacyclobutane complexes, molybdacyclopentane complexes, base-free methylene complexes, and ethylene complexes such as Mo(N-2,6-Cl2C6H3)(CH2=CH2)[rac-Biphen](Et2O). (C) 2004 Elsevier B.V. All rights reserved.