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 field. It was Dr. Alan Longhurst, Director-General of the Atlantic Region, Canada Department of Fisheries and Oceans, who pointed out that our introduction had a far wider audience: in fact, anyone with a scientific background who needs a basic under standing of the physics of the atmosphere and ocean. Dr. D.J. Larner of Reidel agreed, and this book is the result. Since we expended considerable effort to satisfy the demands of the physical chemists, and also discussed the explanations much with our colleagues at home, we expect the reader will find the two parts to be complementary and useful as a unified reference text. On the other hand, since it was designed as background material for a text on air-sea gas exchange and trans port, the more experienced reader will be aware that the picture presented emphasizes transport and exchange processes while it ignores others. No mention is made, for example, of weather forecasting; neither is large-scale ocean modelling considered.