The commercialization of the hydrophohic fibers has necessitated the development of entirely new classes of dyes. This job has been accomplished successfully by the careful tailoring of dye molecules to the chemical and physical requirements of the different fibers.All of the hydrophobic fibers designed for apparel use can be dyed with disperse dyes. Polyamide and acrylic fibers can be dyed with acid dyes. Selected acrylic fibers can be dyed also with basic dyes. The dyeing of hydrophobic fibers with disperse dyes involves solution of the dye in the fiber : electrostatic interactions are involved in the dyeing of these fibers with ionic dyes. The rate of dyeing with both the disperse and the ionic dyes is largely a function of the specific fiber and the molecular size and geometry of the dye molecules. The polarity of a disperse dye plays a major role in determining the affinity of the dye for a specific fiber.Resistance of both the disperse and the ionic dyes to removal from the fiber by aqueous treatments is directly proportional to the affinity of the dye for the fiber and inversely proportional to the rate of dyeing. The washing fastness of the extremely hydrophobic fibers is excellent with all of the dyes which are applicable.The photochemical decomposition of a dye on a fiber is a reaction of the electronically excited dye molecule with the fiber and or other elements of the environment. Hence, light-fastness is a characteristic of the particular dye fiber system. Many correlations of affinity and fastness properties with molecular structure can be drawn.i w E plan to summarize the present status of the (1%-eitig of the hyclrcy>lu~l~ir wnth··tir filx·r,. ~~'hile our emphasis will he on the newer acrytic and polyester nhers. a clearer understanding of the overall devetoptnent is possihte from a simuttaneons consideration of celitilose acetate and the nylon polyamides,------L ntit the advent of the new synthetic fibers, dyeinK involved almost exclusively water-solultle colors. These colors dyed hy a process of ahsurption where the tllwr and dye showed an attraction, or affinity, for each other. The chenncat reactions involved either salt formation between dye ions and ionized filter, as in the case of wool. or intermotecutar assodations, as in the case of cellulose.