Polyolefin thermoplastic elastomers (TPE-Os) are high-performance polyolefins consisting of both plastic and rubber phases. Compared with other thermoplastic elastomers, the TPE-Os possess better chemical stability, transparency, re-processability, and electrical insulation, which renders their broad applications possible. By manipulating chain structures and topologies of the TPE-Os, differently structured polyolefins with improved thermal and mechanical properties have been developed, including ethylene and alpha-olefin random copolymers (POEs), olefin block copolymers (OBCs), comb-shaped polyolefin elastomers (CPOEs), and dynamically cross-linked polyolefin elastomers. Herein, we review the synthesis of the POE, OBC, CPOE, and dynamical cross-linked polyolefin elastomer, including the catalyst systems, polymerization techniques, processes, and kinetic modelling. The characterization of the TPE-Os and the relationships between the TPE-O chain structure, aggregated state, and product performance are discussed. The future development of higher-performance TPE-Os is envisaged.