Nitrifying bacteria convert the most reduced form of soil nitrogen, ammonia, into its most oxidized form, nitrate. In itself, this is important for soil ecosystem function, in controlling losses of soil nitrogen through leaching and denitrification of nitrate. Nitrifiers also contribute to other important processes, including nitrous oxide production, methane oxidation, degradation of organic compounds, and carbon monoxide oxidation. The development of 15N-based techniques has increased significantly our ability to dissect soil nitrogen transformations and their rates, while molecular techniques now enable characterization of soil nitrifier community structure and changes in species composition. These approaches are increasing our understanding of the ecology of soil-nitrifying bacteria and provide the potential for determining relationships between diversity, community structure, and ecosystem function in this important group of organisms.