Passing of drugs across epithelial cell sheets and endothelial cell sheets is an obligatory step in the absorption of a drug. The passing routes of drugs are classified into transcellular and paracellular pathways. The transcellular route has been widely investigated and is used in clinical therapy. In contrast, drug delivery using the paracellular route has never been fully developed. Sodium caprate is the only absorption-enhancer of drugs that uses the paracellular route. Tight junctions (TJs) exist between adjacent cells in epithelial and endothelial cell sheets, and they play a role in sealing the cell sheets. Therefore, we must modulate the TJ barrier for drug delivery using paracellular route. In this review, we describe barriology, including very recent topics, and overview absorption-enhancers from the perspective of barriology.