Delrin acetal resin, a product of DuPont, is formed from the polymerization of formaldehyde. The tightly interlocked helical molecules and high crystallinity result in excellent mechanical properties. Its superiority in tensile strength, stiffness, creep resistance, and fatigue classify it as an "engineering resin', a term used for plastics which can compete with metals in many applications. One of the important characteristics separating Delrin from other engineering plastics such as polyamides (nylons) is its very low water absorption and the small effect of aqueous solutions on its properties. The structure of Delrin is reviewed on several levels: chemical, crystallographic, lamellar, and spherulitic. The relationship of the structure to the properties of moldings of Delrin is discussed.