Epsilon polycaproamide (nylon 6), with more than twenty years' effort behind it in Europe, is now established as a commercial fiber in the U.S. Historically, its development goes back at least sixty years, when it narrowly missed becoming the first truly man-made fiber. Although the molecular structure of nylon 6 is almost identical to that of nylon 6,6, its melting point is lower, probably due to special entropy considerations. Thus nylon 6 permits easier and more economical thermal processing. Thermal properties of the two nylons, even when both are modified with antioxidants, are seen to be similar in many respects, but the 6 type has more resistance to thermal creep at high loads and is more stable with respect to end-group changes. A major domestic producer has developed technology solving the old problem of excessive monomer content without resorting to washing of yarn. Properties, applications, and current developments of various forms of nylon 6 fibers are described. A large potential is shown to exist for expanded use of this versatile fiber.