In this book fundamental features and industrial applications of the direct fluorination of polymers are reviewed. Direct fluorination of polymers (i.e. treatment of a polymer surface with gaseous fluorine and its mixtures) proceeds at room temperature spontaneously and can be considered as a surface modification process. The author of the current book and his co-authors have studied the direct fluorination of more than 20 polymers (polystyrene, polyethyleneterephthalate, poly(2,6-dimethyl-1,4-phenylene oxide), polymethylmethacrylate, low density polyethylene (2 types), high density polyethylene (6 types), polyvinyltrimethylsilane, poly(4-methyl-pentene-1), polyimide MatrimidY 5218, polysulfones, polyetheretherketone, polycarbonatesiloxane, polysulphone-polybuthadiene block-copolymers, polypropylene, PVF, PVDF, etc.). A large variety of experimental methods, such as FTIR spectroscopy, visible and near UV spectroscopy, Electron Spin Resonance spectroscopy, laser interference spectroscopy, refractometry, electron microscopy, method of surface energy measurement, gas-chromatography, method of measurement of permeability of liquids through polymer materials, etc. was applied. Fundamental features of the direct fluorination, such as influence of treatment conditions (composition of the fluorinating mixture, fluorine partial pressure, temperature and fluorination duration) on the rate of formation, chemical composition, density, refraction index and surface energy of the fluorinated layer, kinetics of formation of radicals during fluorination and their termination, texture of fluorinated layer, etc. were studied. On the base of obtained experimental data a theoretical model of the direct fluorination of polymers was developed. It was demonstrated experimentally, that the direct fluorination can be effectively used to enhance commercial properties of polymer articles, such as barrier properties of polymer vessels, bottles and packaging, gas separation properties of polymer membranes and mechanical properties of polymer-based composite materials. Data on a fundamental research and commercial applications provided by other research groups are reviewed.