The performance of standard poly-ethylene terephthalate filaments is in many cases seriously prejudiced by some negative properties of these fibers, such as their nearly complete water repellency, difficult dyeability, tendency to pilling, formation of static, etc. The present paper aimes to recall the methods proposed and realized in order to overcome these shortcomings by modification of the macromolecule. The subject is divided in two groups regarding respectively the modifications for obtaining changes of mechanical properties and shape of the fiber (shrinkage, elasticity, pilling performance, impact resistance, crimping) and of specific properties (dyeing behavior, static load, flame resistance, soiling and soil removal).