Nylons are polymers having amide () linkages. Among this class of polymers, nylon 6 is commercially the most important of the synthetic polyamides available and is used for a variety of purposes, as for example, in the form of fiber in tire cords, apparel, fishing nets, upholstery, hosiery, or in the manufacture of injection and blow-molded objects. There are two common industrial routes to manufacture the polymer from the monomer, ϵ-caprolactam (C1). In the first one, referred to as hydrolytic polymerization, the monomer is mixed with water and is polymerized in the temperature range of 220 to 270°C. The mechanism of polymerization includes three major types of reactions: ring-opening of caprolactam by water, chain growth polymerization, and step growth reaction. Polymers with number-average chain lengths of about 140–180 are usually obtained and these are suitable for use as tire cords, fibers for apparel, as well as resins for molded articles. The second route of polymerization is by the ionic chain growth mechanism, using, for example, initiators like sodium hydroxide, lactamates of alkali metals, and penta-alkyl guanidine. The advantage of this second route lies in that it can be carried out below the melting point (220°C) of nylon and leads to the formation of very high molecular weight polymers. The polymer thus formed is usually employed in making large cast articles. Most nylon 6 is manufactured industrially using the first route, and in view of this, in this review, attention is focused on the design of reactors for nylon 6 through hydrolytic polymerization.