Review article describing the skin as a barrier between the environment and internal milieu, emphasizing its role in preserving local and systemic homeostasis. The article discusses the cutaneous neuroendocrine system, including local equivalents of classical neuroendocrine axes, and explains how skin recognizes environmental signals, produces biologically active compounds, and communicates with immune, neural, endocrine, pigmentary, epidermal, and adnexal systems.