Over the past eight years, early transition metal catalysts for the ring opening metathesis polymerization of cyclic olefins have been developed. These catalysts are simple organometallic complexes containing metal carbon multiple bonds that in most cases polymerize olefins by a living process. These catalysts have been used to prepare a family of near monodispersed and structurally homogeneous polymers. A series of group VIII ROMP catalysts that allow a wide range of functionality to be incorporated into the polymer side chains have recently been prepared. This most important members of this family of complexes are the bisphosphinedihaloruthenium carbene complexes. These catalysts show excellent functional group stability and can be used to prepare well defined telechelic polymers, polyolefins with ordered functionality, and highly functionalized block copolymers.