Molecular transport of substances in the presence or absence of membranes is an ubiquitous phenomenon. Research workers from various disciplines in the biological and phys- ical sciences are actively pursuing problems of transport. One of the important questions arising in numerous biological transport situations concerns the differentiation between passive and active transport. The latter is a more complex phenomenon involving metabolic processes, and the economy of thinking requires that attempts to explain any transport pro- cess from passive mechanisms should be carried as far as pos- sible before invoking more complex mechanisms. A precise de- finition and circumscription of passive transport processes is of crucial importance. The symposium on "Passive permea- bility of cell membranes" was therefore devoted to these pas- sive transport processes and this book presents the proceed- ings of the conference. This symposium, a satellite symposium of the XXV Inter- national Congress of Physiological Sciences at Munich, Ger- many, on July 25-31, 1971, was organized by the Department of Physiology, University of Nijmegen, Nijmegen, The Nether- lands. It was held at the Hilton Hotel in Rotterdam on July 20-22, 1971. The meeting was made possible by generous fi- nancial support from the Dutch Ministry of Education and Science (Ministerie van Onderwijs en Wetenschappenl and from Boehringer Mannheim N. V., Ciba N. V., Essex (Nederland) N. V., Gist-Brocades N. V., Hoffmann - La Roche N. V., Merck, Sharp and Dohme Nederland N. V., N. V. Philips, and Unilever N.