In this lecture, the structure of the epithelium and the epithelial fluid transport will be outlined as introduction. Next, the lecture will explain the basic mechanism by which major electrolytes and water are absorbed into or secreted from epithelial cells by the driving force of the sodium pump. In addition, the molecular mechanism of the paracellular transport, which has recently been clarified, will be described. Then, the mechanism of fluid transport in renal and small intestinal epithelia will be explained.