This monograph, ""Air-Water Mass Transfer: Selected Papers from the Second International Symposium on Gas Transfer at Water Surfaces"", consists of papers presented at the Symposium which was held in Minneapolis, Minnesota, September 11-14, 1990. The topics discussed in these papers include: fundamentals of physical phenomenon and modeling; laboratory and field measurement techniques; and applications of air-water mass transfer technology. The text begins with a discussion of some of the physical characteristics of and chemical effects on gas transfer. Next, comes an exploration of physical phenomena, such as turbulence, wind, waves, convection, density stratification, and bubbles, including the development of numerical, conceptual, semi-empirical, and empirical models. This is followed by 13 papers concerning different laboratory and field measurement techniques, such as remote sensing and gas tracer techniques. Finally, the majority of the papers discuss various applications of gas transfer to streams and rivers, lakes and reservoirs, hydraulic structures, water and wastewater treatment, seas and oceans, and artificial aeration.