31P NMR has been shown to be a useful method for the study of microbial metabolism in vivo. This is mainly due to the fact that a variety of important metabolites, such as ATP, are phosphorylated, and that the31P nucleus has a 100% natural abundance. The concentrations of phosphorylated metabolites and time-dependent changes in metabolism can be monitored non-invasively by31P NMR. Furthermore, the resonance positions of some of these molecules are sensitive to the local ionic environment and, thus, can be used to estimate cellular pH. Saturation-transfer31P NMR allows the measurement of fluxes through certain enzymatic reactions under steady-state conditions. Since such parameters are measured in the intact cell.31P NMR has led to new insights into the regulation of cellular processes.