The analysis of intracellular metabolite concentrations is of basic importance for metabolic engineering of microorganisms. In vivo NMR-spectroscopy as a non-invasive technique to measure intracellular metabolite concentrations and rapid sampling devices as invasive techniques are reviewed. The methods are discussed from a reaction engineering point of view. The objective is to obtain intracellular concentration data under well defined physiological conditions in balanced steady state and defined transitional states as well. Application examples are given for a membrane-cyclone-reactor configuration designed to achieve high signal sensitivity with in vivo 31P-NMR and 13C-NMR spectroscopy as well as for a sampling tube device designed for high sampling rates (2s-1). This sampling device enables the measurement of dynamic metabolite profiles at a time scale of a few seconds.