Ammonia volatilization from flooded soils has been studied for over half a century. In reviewing the literature on this subject, it becomes clear that there is no consensus on the importance given to this loss mechanism. In part, the differences of opinion can be explained by the fact that ammonia losses were studied in different environments, but to a great extent it seems due to the wide diversity of techniques used to study this loss mechanism. The many factors that influence ammonia volatilization from flooded soils are chemical, biological, and environmental in nature. These various factors are reviewed in depth and discussed with respect to their implications for measurement techniques and for soil, fertilizer, and water management. The major objective of this paper is to familiarize the reader with the most current developments in thinking about the mechanisms and extent of ammonia loss and hopefully to stimulate meaningful research on ammonia volatilization from flooded soils. Such research should be conducted in a wide range of agroclimatic conditions utilizing measurement techniques that are valid or for which the limitations are clearly understood. A better appreciation for the importance of ammonia volatilization will provide the impetus to research and development in fertilizer technology and management aimed at preventing such losses.