In-cell NMR is a useful approach for obtaining structural and dynamics information of the proteins within living cells at atomic resolution, but the rapid increase in the dead cell population during NMR measurements has limited the applications of various NMR experiments that require longer measurement times. To overcome this problem, bioreactor systems have been developed, in which fresh culture medium is supplied to cells encapsulated in gels inside the NMR tube, thereby maintaining physiological intracellular conditions during NMR measurements. This chapter describes the methodological detail of the bioreactor system, and its applications for in-cell NMR observations, especially the protein–protein interactions and real-time observations of intracellular events.