1IntroductionSince the early nineties and the discovery of structurally well-defined catalysts for alkene metathesis by Schrock and Grubbs, the field of organic synthesis has undergone an exciting transformation.1Through catalytic olefin metathesis, chemists can now efficiently synthesize an impressive range of molecules that only a decade ago required significantly longer and tedious routes. The primary reason for the success of olefin metathesis is the development of increasingly efficient and selective catalysts.In this perspective article, we outline the development of a class of Ru carbenes represented by1–3, that are emerging as increasingly popular metathesis catalysts as a result of their unique properties; three representative transformations that are most effectively promoted by this class of Ru complexes are depicted inScheme 1(see below for additional details). The story begins with the serendipitous discovery of Ru-based complex1through a set of experiments intended to shed light on the mechanism of various metal-catalyzed ring-opening/ring-closing metathesis (ROM/RCM) reactions.2Subsequent studies resulted in the availability of non-phosphine Ru carbene2and its chiral analogue3as catalysts that are recyclable, stable to air or moisture, and operate in the presence of a wide range of common organic functionalities.Ru-based complexes bearing a bidentate styrene ether ligand serve as effective and practical olefin metathesis catalysts. The reactions shown can only be catalyzed or promoted with high efficiency by this class of Ru complexes. Mes = 2,4,6-Me3C6H2.