This paper gives a review of scientific advances in the understanding of the mechanism of fiber structure development in the high-speed melt spinning process. Research in this field has been active since the late 1970s. Particular attention is paid to the molecular orientation and orientation-induced crystallization occurring in the high-speed spinning process of poly(ethylene terephthalate), polyamides, and some other types of polymers. Characteristics of crystalline morphology developed in the spin line and evolution of structural variation in the cross-section of the fiber are also discussed. On-line measurement of the spin line revealed that fiber structure development is accompanied with neck-like deformation. Detailed behavior of the neck-like deformation such as diameter profile, necking temperature, and necking draw ratio are presented, and the relation between the neck-like deformation and the structure development is also discussed. (C) 2002 John Wiley & Sons, Inc.