Book excerpt: A full review of the latest research findings on microbes involved in conventional aerobic nitrification, anaerobic ammonia oxidation, and related processes. • Examines the four principal groups of nitrifying microbes including conventional aerobic bacterial ammonia oxidizers, recently discovered aerobic archaeal ammonia oxidizers, anaerobic ammonia-oxidizing planctomycetes, and nitrite-oxidizing bacteria. • Provides current information on the ecology, phylogeny, biochemistry, molecular biology, and genomics of each group of microbes. • Discusses the latest industrial applications of nitrification and anammox processes, and explores the ecology of nitrification in marine, freshwater, soil, and wastewater environments.