The transfer across the surface of environmental waters is of interest as an important phase in the geophysical and natural biochemical cycles of numer­ ous substances; indeed it governs the transition, one way or the other, between the dissolved state in the water and the gaseous state in the atmo­ sphere. Especially with increasing population and industrialization, gas transfer at water surfaces has become a critical factor in the understanding of the various pathways of wastes in the environment and of their engineering management. This interfacial mass transfer is, by its very nature, highly complex. The air and the water are usually in turbulent motion, and the interface between them is irregular, and disturbed by waves, sometimes accompanied by breaking, spray and bubble formation. Thus the transfer involves a wide variety of physical phenomena occurring over a wide range of scales. As a consequence, scientists and engineers from diverse disciplines and problem areas, have approached the problem, often with greatly differing analytical and experimental techniques and methodologies.