The state-of-the-art on processes by which earthworms emit nitrous oxide (N2O) and dinitrogen (N2) are presented by this review. The emission of these nitrogenous gases appears to be primarily due to soil-derived denitrifying bacteria when they are subjected to the unique in situ conditions of the earthworm gut. The in situ factors of the earthworm gut that contribute to the stimulation of ingested denitrifiers include anoxia, high quality organic carbon, and nitrate or nitrite. Functional gene analyses corroborate cultivation-based studies that indicate denitrifiers of soil and gut microbial biomes are phylogenetically similar. In addition to effecting the in vivo emission of nitrogenous gases, the unique microenvironment of the earthworm gut might also affect the fitness and diversity of certain members of the soil microbial biome.