The pH of the skin follows a sharp gradient across the stratum corneum (SC), which is suspected to play an important role in controlling the enzymatic activities involved in cellular metabolism and renewal. This gradient is maintained by several systems, such as sweat and sebum secretion and degradation as well as cellular metabolism. At the surface of the skin, what is measured is in fact an apparent skin pH due to extracted material from the SC diffusing into water applied at the surface. pH values recorded at the surface of a semi hydrophobic milieu such as the SC should be interpreted with great caution because it is obvious that hydrogen ions are not in a pure solution at the surface of the skin. For a correct measurement of skin surface pH, it is recommended to follow all practical operating conditions. Care must be taken in identifying the skin site, healthy controls (age, gender, skin type), the time of day of the measurement and the environmental conditions. Also, subjects should receive precise instructions before the test, mainly in terms of hygiene procedure or use of topical products. The interpretation of data should not overlook the fact that even small differences in pH may reflect significant modifications at the molecular level. Although it is usually agreed that the pH of the skin surface may influence the cutaneous microflora, much remains to be learnt about the role of the acid mantle of the skin with regard to defensins and other protective mechanisms.