Discussion of the importance of considering the "ecological diversity" of the skin when considering a product for topical use. Substances should be used carefully on the skin as many compounds may cause an allergic response or act as feeding grounds for some of the 1000 species of bacteria which inhabit the skin. The allergic response was outlined, including terms like antigen, allergen, antibodies, and histamine. The overgrowth of bacteria or yeast when topical cosmetics are used needs to be addressed when manufacturing these products. A list of 26 allergenic substances will be presented and briefly discussed.