Technical Abstract: Cotton fibers are natural plant products and their end-use qualities depend on their stages of development. In general, the quantity of natural fiber cellulose I (ß 1'4 linked glucose residues) increases rapidly, thus it leads to compositional, structural, and physical attribute variations among the fibers with shorter and longer growth periods. This article discusses recent progress in applying Fourier transform infrared (FTIR) spectroscopic technique to characterize these differences, to discriminate immature fibers from mature fibers, to assess fiber maturity and crystallinity, and also to unravel the band assignments in crystalline and amorphous celluloses. The results were achieved through the use of various strategies, including wet chemical analysis, principal component analysis (PCA), simple algorithm development, two-dimensional correlation analysis, and other independent fiber tests. Of particular interest is that, in general, immature fibers might have the characteristics of less than 21-28 dpa, MIR < 0.58 (in the maturity range of 0 to 1.0) and CIIR < 42% (in the crystallinity range of 0 to 100%).