One of the recurring debates in cosmetics development relates to the penetration of ingredients. How do manufacturers ensure optimal penetration for efficacy? What is the optimal penetration? How can water, lipophilic molecules, liposomes or nanoparticles penetrate through the stratum corneum (SC) and reach deep into the epidermis and below? This overview takes a somewhat different approach to the question of penetration by focusing on the SC, the first layer implicated by application of topical products to the skin. It also investigates how molecules are able to penetrate this outermost layer and stresses the fact that even if they penetrate no deeper, they play an essential role in skin health and youthfulness.