Given the spectrum of diseases affecting surface epithelia, it is evident that keratinocytes are'at risk'from a range of aetiological agents. However, numerous protective factors are also present and whereas Langerhans and melanocyte cell populations are well recognized, insufficient regard has been paid to the potential of keratinocytes themselves in providing protective mechanisms, apart that is, from their abilities to undergo phagocytosis and to desquamate. Recent concepts of the proliferative organization of epidermis and oral epithelium have focussed attention on the role of a minority stem cell population in epithelial homeostasis. Whilst the characterization of stem cell attributes has led to the identification of features that cati be considered'protective'in the context of carcinogenesis, the non-stem cells (the majority of the basal and all suprabasal cells) can also be regarded as contributing a measure of protection. This paper describes some recent concepts of epithelial homeostasis and considers mechanisms that can theoretically be termed'protective'resulting from the organization of keratinocytes into a proliferative hierarchy.