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Catalysts contribute to the efficient production of chemicals and materials in almost all processes in the chemical industry. The polyolefin industry is one prominent example of the importance of catalysts. The discovery of Ziegler-Natta catalysts in the 1950s resulted in the production of high-density polyethylenes (PEs) and isotactic polypropylenes (iPPs). Since then, further catalyst development has led to the production of a new series of polyolefins, including linear low-density PEs, amorphous ethylene/1-butene copolymers, ethylene/propylene/diene elastomers, and syndiotactic PPs (sPPs). Polyolefins are now the most important and the most produced synthetic polymers. This Account describes a family of next-generation olefin polymerization catalysts (FI catalysts) that are currently being used in the commercial production of value-added olefin-based materials. An FI catalyst is a heteroatom-coordinated early transition metal complex that combines a pair of nonsymmetric phenoxy-imine [O(-), N] chelating ligands with a group 4 transition metal. The catalytically active species derived from FI catalysts is highly electrophilic and can assume up to five isomeric structures based on the coordination of the phenoxy-imine ligand. In addition, the accessibility of the ligands of the FI catalysts and their amenability to modification offers an opportunity for the design of diverse catalytic structures. FI catalysts exhibit many unique chemical characteristics: precise control over chain transfers (including highly controlled living ethylene and propylene polymerizations), extremely high selectivity for ethylene, high functional group tolerance, MAO- and borate-free polymerization catalysis, significant morphology polymer formation, controlled multimodal behavior, high incorporation ability for higher alpha-olefins and norbornene, and highly syndiospecific and isospecific polymerizations of both propylene and styrene. These reactions also occur with very high catalyst efficiency. The reaction products include a wide variety of unique olefin-based materials, many of which were previously unavailable via other means of polymerization. We have produced selective vinyl- and Al-terminated PEs, ultrahigh molecular weight linear PEs, regio- and stereoirregular high molecular weight poly(higher alpha-olefin)s, ethylene- and propylene-based telechelic polymers, a wide array of polyolefinic block copolymers from ethylene, propylene, and higher alpha-olefins, and ultrafine noncoherent PE particles. FI catalysts are important from the organometallic, catalytic, and polymer science points of view, and the chemical industry is now using them for the production of value-added olefin-based materials. We anticipate that future research on FI catalysts will produce additional olefin-based materials with unique architectures and material properties and will offer scientists the chance to further study olefin polymerization catalysis and related reaction mechanisms.