This review examines photo-yellowing of wool with emphasis on fluorescent whiteners. Heat and alkali treatments also contribute to yellowing and oxidative rather than reductive bleaching leads to discoloration. Wavelengths between 385-475 nanometers bleach wool whereas yellowing occurs and worsens at wavelengths of 380 nanometers and under. Fluorescent whiteners aggravate yellowing as these absorb at between 360 to 400 nanometers. Photo-oxidation is sensitized by the whiteners. Lightfastness ratings obtained by British Standard Method against blue standards in the Xenotest 150 show that water plays a rôle. Samples bleached with peroxide, treated with whiteners and control all yellow more when wet. Some loss in amino acid residues occurs during yellowing. It is also possible that degradation products hydrolyzed by these amino acids may be reponsible for yellowing. Further hypothesis suggests that yellowing results from the degradation of aliphatic residues by aromatic amino acid residues. Treatment of wool by reaction of methylisothiocyanate in dimethylformamide has limited ability to inhibit yellowing as does impregnation with a mixture of thiourea and formaldehyde followed by thermal curing and impregnation with thioglycollic acid and related compounds. This is a reference for the degradation of wool.