This Account describes our research related to the development of molecular catalysts for solution phase olefin polymerization. Specifically, a series of constrained geometry and nonmetallocene (imino-amido-type) complexes were developed for high temperature olefin polymerization reactions. We have discovered many highly active catalysts that are capable of operating at temperatures above 120 °C and producing copolymers with a useful range of molecular weights (from medium to ultrahigh depending on precatalyst identity and polymerization conditions) and α-olefin incorporation capability. Constrained geometry catalysts (CGCs) exhibit very high activities and are capable of producing a variety of copolymers including ethylene-propylene and ethylene-1-octene copolymers at high reactor temperatures. Importantly, CGCs have much higher reactivity toward α-olefins than classical Ziegler-Natta catalysts, thus allowing for the production of copolymers with any desired level of comonomer. In search of catalysts with improved performance, we discovered 3-amino-substituted indenyl-based CGCs that exhibit the highest activity and produce copolymers with the highest molecular weight within this family of catalysts. Phenanthrenyl-based CGCs were found to be outstanding catalysts for the effective production of high styrene content ethylene-styrene copolymers under industrially relevant conditions. In contrast to CGC ligands, imino-amido-type ligands are bidentate and monoionic, leading to the use of trialkyl group IV precatalysts. The thermal instability of imino-amido complexes was addressed by the development of imino-enamido and amidoquinoline complexes, which are not only thermally very robust, but also produce copolymers with higher molecular weights, and exhibit improved α-olefin incorporation. Imido-amido and imino-enamido catalysts undergo facile chain transfer reactions with metal alkyls, as evidenced by a sharp decrease in polymer molecular weight when the polymerization reactions were conducted in the presence of diethylzinc, an essential requirement for use in the production of olefin block copolymers via chain shuttling polymerization. Overall, the excellent characteristics of imino-amido-type catalysts, including high catalytic activities and ultrahigh molecular weight capabilities, make them good candidates for high temperature syntheses of block and random ethylene-α-olefin copolymers. Additionally, trialkyl imino-enamido complexes react quickly with various protic and unsaturated organic fragments, leading to a library of dialkyl precatalysts that, in several instances, resulted in superior catalysts. In conjunction with the development of transition metal catalysts, we also synthesized and evaluated activators for olefin polymerization. We found, for example, that, when conducted in coordinating solvents, the reaction between aluminum alkyls and tris(pentafluorophenyl)borane leads to the exclusive formation of alumenium borates, which are excellent activators for CGC complexes. Additionally, we developed a series of highly effective new activators featuring a very weakly coordinating anion composed of two Lewis acids coordinated to an imidazole fragment.