Degradation of silk occurs mainly through chemical bond scission reactions involving oxidation and/or hydrolysis. Degradation is initiated and accelerated by both heat and light, leading to the production of free radicals and peroxides. Thermal degradation occurs when bond dissociation energies are exceeded. Photodegradation occurs when visible or ultraviolet light is absorbed. Chromophonic or light absorbing groups must be present; ultraviolet (UV) light of wavelengths less than 400 mm are sufficient to bring about dissociation into free radicals. Impurities can reduce both thermal- and photostability. Describes factors that contribute to, as well as those inhibiting, silk deterioration. In the author's research experience, application of Cyasorb 531/Tinuvin 765 in equal ratios of 2:2 to silk produced the best protection of new silk against heat and light degradation.