What should we know about the skin as an application site for (not only) dermo-cosmetic products? The human skin can undoubtedly be regarded as the largest organ of the human body. This multi-layered selectively permeable tissue protects internal organs and other tissues from damage and is in direct contact with the surrounding environment. Part of this environment includes various substances, medicinal preparations, medical devices, and cosmetics. This article highlights selected facts regarding human skin's structure, significance, and role in local/topical product application. In addition to the overall condition of the skin (healthy vs. diseased), the structure of epidermis/stratum corneum and the pH of the skin surface are key factors influencing the effectiveness of (not only) cosmetic products applied to the skin.