Polyolens constitute the most widely used group of thermoplastics, often referred to as commodity thermoplastics. They are prepared by polymerization of simple olens such as ethylene, propylene, butenes, isoprenes, and pentenes, as well as their copolymers, whereas they are the only class of macromolecules that can be produced catalytically with precise control of stereochemistry and, to a large extent, of (co)monomer sequence distribution. The term polyolens derives from “oil-like” and refers to the oily or waxy feel that these materials have. They consist only of carbon and hydrogen atoms and are nonaromatic. An inherent characteristic common to all polyolens is a nonpolar, nonporous, low-energy surface that is not receptive to inks and lacquers without special oxidative pretreatment. Polyolen-based materials can be tailor-made for a wide range of applications: from rigid thermoplastics to high-performance elastomers. These vastly different properties are achieved by a variety of molecular structures, whose common features are low cost, excellent performance, long life cycle, and ease of recycling. Although polyolens represent one of the oldest (if not the oldest) families of thermoplastic polymers, they are still characterized by innovations that provide new applications via stepwise and continuous technology renewal that reduce the eco-footprint during manufacture and use. The two most important and common polyolens are polyethylene and polypropylene, and they are very popular due to their low cost and wide range of applications.