After the pioneering studies by Ussing and co-workers, studies of epithelial Nael transport have come a long way. The first phase of the phenomenological description of the cell as a black box has been follow­ ed by studies of cellular mechanisms, the interplay of the different trans­ port components, and the mechanisms of regulation. A broad spectrum of methods has been applied to many epithelia in a variety of species. For the individual epithelia transport schemes have been proposed, and, at this point I think it is appropriate to take a pause and search for elements common to several epithelia. This aspect triggered the publica­ tion of this book, and in fact the various chapters emphasize that the funetional eomponents, expressed in the various epithelia, are not in­ finite in number, but they occur in epithelia which are separated in evolu­ tion by several hundred million years. The authors come both from the field of veterinary and human physiology as weIl as from biology. In my opinion, the close contacts and eollaborations between physiologists and biologists have been essential for the progress in this field. I wish to thank all authors for their con­ tributions, and I hope that the reader will appreciate this collection of up to date reviews on epithelia in nonvertebrates and vertebrates.