Polyolefins have dominated materials technology over the last century and remain among the most used materials, despite environmental problems have emerged and their public perception deteriorated. Polyolefin thermoplastic elastomers (P-TPEs) are a particular class of polyolefins: they can be easily molded and extruded like thermoplastics, featuring at the same time elastic properties. P-TPE advantages over thermoset rubbers are their mechanical flexibility, lightweight, easy processability, and recyclability, fulfilling the requirements for a sustainable circular economy of the next plastic generation. This review describes the recent developments based on a historical account and summarize major breakthroughs with the focus on P-TPEs obtained from olefin insertion and metathesis (co)polymerization catalyzed by molecular transition metal complexes. A survey on blocky, branched and grafted polyolefin microstructures accessible is presented and illustrated with manifold examples. The achievements reported in the field pave the way toward the synthesis of smart materials, such as shape memory, self-healing and polar functionalized P-TPEs that store temporary shapes and integrate molecular level functionality. These advanced materials may find application in multilayer processing technologies for innovative food packaging and coatings, commonly requiring the polymer to have multiple functions, and special applications such as long-term biomedical equipment and tissue engineering, 3D printing, flexible electronic energy storage, separation membranes, photonic band gap materials, and forth on. Issues of olefin polymerization catalysis, future challenges and opportunities for recyclable high performance and lasting P-TPEs are addressed. Display omitted