The transport of water across biological membranes is a process of considerable theoretical and practical importance. Classical interpretations of the mechanism of this transport have centered around the hypothesis of aqueous membrane pores. However, more recent evidence indicates that the possible presence of unstirred layers adjacent to membranes must be taken into account. In one sense, the unstirred layers may be considered a vexing technical problem. In these terms, they represent a problem of potential clinical relevance, eg, in the design of more efficient hemodialysis machines. Alternatively, it is not unreasonable to speculate that unstirred layers may result, at least in part, in alterations in the properties of water which result, for example, from hydrophobic interactions between membrane interfaces and vicinal lamellae of water. Considered in this context, unstirred layer phenomenology may be of critical significance, not only to an appreciation of water flows in membranes, but also for the elucidation of the molecular effects of hormones such as antidiuretic hormone (ADH), and, finally, in the rational design of drugs which modify membrane transport processes in a clinically useful way.