Tight junction biology has many important applications, from improving knowledge of diseases characterized by the loss of epithelial or endothelial tissue barrier function to providing a mechanistic basis for improving non-invasive drug delivery. A variety of chemical and molecular biological tools have been developed, including high throughput cell-based screening of molecular libraries that facilitate discovery of novel peptides and lipids to modulate tight junction function safely and reversibly. Further development of novel tight junction modulating excipients necessitates consideration of physicochemical, toxicological, pharmaceutical and regulatory issues.