The two chapters of this book originally appeared in "Air- Sea Exchange of Gases and Particles", edited by P.S. Liss and W.G.N. Slinn. We wrote them as a general introduction to the physical processes in the atmosphere and ocean which govern the transport of gases and particles in and between the two media. Our audience was to be graduate students in physical chemistry of air and sea, and research workers wishing to get started in this or a related...