Polyolefins, the polymers synthesized from olefinic monomers, are the major commercial thermoplastics. They are also important elastomers and the components of major thermoplastic elastomers. Polyethylene and polypropylene in volume are the two largest thermoplastics and are fabricated into filaments, films, and moldings. For the first time, the reader will find a comprehensive description of the structuring of molten polyolefins in fiber, film and molding processes with correlation between structural order such as crystalline unit cell, polymorphic effect and orientation, and processing parameters. Contents: Origins of Polyolefins. Characterization Methods. Crystallography of Polyolefins. Single Crystals: Structural Hierarchy and Morphology. Spherulites and Quiescent Crystallization. Polyolefin Copolymers and Blends. Polymer Melt Processing, Rheological Properties, and Orientation in Flowing Polymer Melts. Melt Spinning. Film Processing and Profile Extrusion. Molding. Mechanical Properties of Polyolefins.