This chapter provides a comprehensive examination of polyolefins, the most widely used class of thermoplastics globally, with an emphasis on their structure, processing, commercial relevance, and sustainability challenges. It begins with an overview of the molecular diversity, processing techniques, and key properties of polyolefins such as polyethylene (PE) and polypropylene (PP). The chapter traces their historical development, industrial impact, and market dominance. It then shifts focus to the growing environmental concerns related to their fossil-derived origins, persistence in the environment, and contribution to plastic pollution and microplastics. Various strategies to enhance polyolefin sustainability are explored, including mechanical and chemical recycling, compatibilization, and the incorporation of renewable feedstocks and fillers. Detailed mechanisms of degradation—thermal, mechanical—are also discussed in relation to processing and recyclability. The chapter concludes by framing polyolefins within the broader context of circular economy goals, highlighting the urgent need for innovation in materials science, infrastructure, and policy to enable sustainable lifecycles for these essential yet environmentally persistent polymers.