A number of recently developed techniques which may be used for rapid quantification of total microbial activity or of specific microorganisms involved in metal corrosion are reviewed. These include viable counts using dip slides, microscope methods (epifluorescence), analysis of cell components (adenosine triphosphate, ergosterol, phospholipids), immunological methods (fluorescent antibody staining, enzyme-linked immunosorbent assay), activity measurements (respiration, esterases, sulfate reduction) and the use of gene probes. The methods are discussed with reference to their simplicity, sensitivity, economics, and applications.