This article discusses the most fundamental building-block level, atomic level, molecular considerations, intermolecular structures, and supermolecular issues. It contains a table that lists the structures and properties of selected commodity and engineering plastics. An explanation of important thermal and mechanical properties of solid engineering plastics and molten engineering plastics is provided. The article describes the effects of structure on thermal and mechanical properties. It discusses the chemical, optical, and electrical properties of engineering plastics and commodity plastics. The article outlines the most common plastics manufacturing processes. The factors that must be considered when processing engineering thermoplastics are discussed. These include melt viscosity and melt strength; crystallization; orientation, die swell, shrinkage, and molded-in stress; polymer degradation; and polymer blends.