The scope of this book was preempted and precipitated by the pioneering studies of Ussing and co-workers. The first phase of the phenomenological description of the cell as a black box has been followed by studies of cellular mechanisms, the interplay of the different transport components, and the mechanisms of regulation which will be covered in the individual chapters. For the individual epithelia transport schemes that have been proposed, I think it is appropriate to take pause and search for elements common to several epithelia. This aspect triggered the common theme idea for this book, and in fact the various chapters emphasize that the functional transporter components, expressed in the various epithelia, are not infinite in number, but they occur in epithelia which are separated in evolution by several hundred million years.