The skin barrier is physically located in the upper part of the epidermis below the actual skin surface. It is formed by keratinocytes (skin cells), intercellular lipids and natural moisturizing factors (NMFs); its main function is to retain moisture and to protect the body from the intrusion of foreign materials. The skin barrier is continually regenerated following the skin renewal cycle. The environmental influence to this process is significant. The skin barrier condition directly affects the skin's health. When the skin barrier is normal, it can effectively maintain skin moisture and suppleness, as well as efficiently prevent the occurrence of skin diseases. When skin barrier function is disrupted, the skin can easily be penetrated by foreign materials, which can lead to "sensitive skin" conditions. When the barrier is further damaged, the skin begins to lose its moisture retention capacity, so it suffers from "dry skin" symptoms. When the barrier is seriously damaged, various skin disorders or disease can occur, such as eczema, atopic dermatitis or psoriasis.