The healthy stratum corneum allows optimum permeability of water and provides the first line of defense against pathogenic and environmental assaults. The barrier functions of the stratum corneum are interrelated, coregulated, and interdependent. Research has demonstrated that three lipid species, which usually comprise 10% of the stratum corneum, are crucial to both its structure and its function; these must be present in sufficient quantities and in the correct proportions to provide optimum barrier function. The clinical implications of how the skin barrier works--and is supported and restored--can be seen in the current and emerging understanding of atopic dermatitis management.