ARLANXEO Elastomers has developed and commercialized Keltan ACE (TM) technology, a class of half-sandwich cyclopentadienyl kappa(1)-amidinate metal complexes, which are extremely active for the production of first-class ethylene/propylene/diene copolymers (EPDM). In this review, the development and some of the key features of the Keltan ACE (TM) catalyst system are presented. Many different ACE catalysts have been synthesized over the past years, including bridged and bimetallic catalysts. With Keltan ACE (TM), a complete range of EPDM products with similar polymer characteristics as their Ziegler-Natta (ZN) counterparts can be produced, including variations containing very high 5-ethylidene-2-norbornene (ENB) contents, controlled long chain branching, very high molecular weight, as well as oil-extended products. Moreover, other EPDM structures can be polymerized. The Keltan ACE (TM) catalyst technology also allows the production of EPDMs with very high amounts of dicyclopentadiene (DCPD) or 5-vinyl 2-norbornene (VNB) without exc1essive gelation and reactor fouling, that is, products that cannot or are extremely difficult to obtain via classical ZN catalysis. In a next step, high-VNB-EPDM can be postreactor modified, for example, via metathesis chemistry. In addition, EPDM polymers with a very broad or even bimodal molecular weight distribution can be obtained in a single reactor with certain ACE catalyst structures at particular activator/precatalyst ratios. (C) 2017 Wiley Periodicals, Inc.