As-manafactured polymeric fibers are oriented semi-crystalline structures in which the macromolecules are rarely in their equilibrium state. Further instabilities are imparted when the fibers are converted to yams and the yarns to fabrics. Heat setting is an important industrial process, since it rids them of their instabilities. This review article first considers aspects that are relevant to the broad area of heat setting, viz., the origin of instability, the durability of set, thermal and dynamic mechanical transitions, thermodynamic basis of setting, shrinkage and shrinkage stress, and characterization of the degree of set. The heat setting of major thermoplastic fibers, viz., polyester, polyamide, polyacrylonitrile, polypropylene, and polyethylene has been described in terms of the conditions of heat setting, the structural and morphological changes that occur on heat setting, and the changes in their physical properties. It is shown that heat setting affects such important properties as stress-strain and recovery behavior, dye-uptake, optical properties, and thermal properties. The structural and morphological changes have been described in terms of changes in crystallinity, crystal size and their size distribution, crystal defects, crystal orientation, the nature of the amorphous phase and its orientation, the coupling of the crystalline and amorphous phases, etc. The structural basis of property changes is then discussed. (C) 2002 John Wiley & Sons, Inc.