The “living” polymerizations possible with well-defined molybdenum and tungsten olefin metathesis catalysts are presented in this 1990 review article from Richard Schrock. Focusing mostly on ring-opening metathesis polymerization of norbornenes and norbornadienes, Schrock offers a concise but comprehensive examination of polymer stereochemistry and its relationship with the ROMP mechanism. Olefin substitution and catalyst variants are directly tied to changes in polymer tacticity and polydispersity. The production of conjugated polyenes and polymerization of acetylene is also discussed. Since this review’s publication, the use of these catalysts has advanced in many ways (and has been re-summarized by a number of researchers, including Schrock himself), but this article is still commonly cited when discussing the molybdenum and tungsten alkylidenes.