Publisher Summary This chapter explains in detail various thermoplastic resins, which are any synthetic material that can be molded when hot but becomes hard on cooling. Polyethylene is a broad family with versatile properties that depend on the three main polymerization processes used. Metallocene catalysis polymerization is the most recent technique, growing fast to produce a consistent, uniform distribution of molecular weight resulting in enhanced toughness, impact and puncture strengths, better cold behavior and optical properties. The copolymer blocks present a good balance between rigidity and impact strength that depends on the importance of the blocks and their distribution. Chlorinated polyethylenes present a good set of properties for a highly flexible material. The study reports general assessments of behaviour after prolonged immersion in a range of chemicals at room temperature for given grades, which are not representative of all the chlorinated polyethylenes because of the variation of the chemical resistance when the chlorine content varies and the broad possibilities for formulating the compounds: stabilizers, modifying agents, fillers and so on. These general indications should be verified by consultation with the producer of the selected grades and by tests under operating conditions.