High-Density Polyethylene (HDPE) is a tough, durable thermoplastic derived from ethylene monomers, notable for its high crystallinity and linear polymer structure. With a density of 0.94–0.97 g/cm³, HDPE is more rigid and stronger than LDPE, while maintaining excellent moisture and chemical resistance. It has a higher melting point (120–135°C) and superior tensile strength, which makes it ideal for heavy-duty applications such as pipes, containers, bottles, fuel tanks, and industrial packaging. HDPE is opaque with a milky appearance, reflecting its crystalline structure, and is less flexible than LDPE, but far more durable. It is also resistant to impact, stress cracking, and wear, making it suitable for long-lasting applications. HDPE is classified as recycling code #2 and is one of the most commonly recycled plastics worldwide. Despite being non-biodegradable, its recyclability and strength-to-weight ratio make it a preferred choice in construction, consumer goods, and packaging industries. HDPE is lightweight yet tough, easy to process by extrusion and molding, and offers excellent electrical insulation properties. It is sensitive to UV degradation but can be stabilized with additives for outdoor applications. As a globally dominant polymer, HDPE is valued for its balance of strength, durability, and versatility.