This book was written by many outstanding investigators who have spent decades to study different aspects of blood‑tissue barrier function. They have summarized some of the latest and fascinating development in their fields of research including the blood‑brain barrier, the blood‑retinal barrier, the gut barrier, the blood‑biliary barrier, the blood‑follicle barrier, the blood‑epididymis barrier, the blood‑testis barrier, the tight junction barrier in general as well as barriers in the female reproductive tract. Included are also chapters that focus on topics that are physiologically applicable to all blood‑tissue barriers. Many of these chapters also include information on specific human diseases, such as pathological changes of the gut barrier that cause bowel disorders resulting from inflammation of the epithelial lining in the intestine, and infertility in men as a result of disruption of the blood‑epididymal and/or blood‑testis barriers; and on new therapeutic approaches (e.g., drug delivery across the blood‑brain and the blood‑retinal barriers).