This review summarizes the state of microscale in situ analyses of nitrification. Micro-sensors and fluorescence in situ hybridization (FISH) have both become important methods for microbial ecology. In combination, they have offered a variety of excit-ing insights into structure and activity of microbial communities, including nitrifying biofilms, aggregates, and sediments. Several levels of information can be obtained: (a) identification of microenvironments or environmental conditions (such as concentra-tions of oxygen, ammonium, nitrite) favored by particular microorganisms; (b) qualita-tive evidence of certain metabolic activities of particular microorganisms, e.g., nitrite oxidation by Nitrospira sp.; and (c) quantification of cell-specific reaction rates and estimation of the in situ kinetics of uncultivated microorganisms. Potential, limitations, and future perspectives of such combined microsensor/FISH approaches for the study of nitrification are discussed.