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The term "skin barrier" is somewhat ambiguous: it is often used to indicate essentially the "stratum corneum", the uppermost layer of the skin. As the word "barrier" implies not only a physical obstacle (e.g. dead corneocytes) but also a defense system, we should also include the viable epidermis in consideration of skin barrier/defense. I will first review some of the recent developments in cosmetic research on ceramides, on fatty acid derivatives, on enzyme systems in the stratum corneum and then take a look at the latest findings that highlight the role of epidermal structures and functions to defend the skin against damaging influences. The "inside" and "outside" terminology has two meanings: topical application of ceramides, for instance, is considered an "outside" approach; whereas using specific ingredients that stimulate ceramide synthesis in the skin can be called an "inside" effect. Furthermore, oral nutritional supplements, increasingly popular in what is called "oral cosmetics" can also bring substantiated benefits to the skin barrier from the "inside". Examples from academic and industrial research will be given.
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