Microreactor technology has gained significant popularity in:the chemical and process industry in the past decade. The development of microreactors either as innovative production units; for chemical synthesis or as promising laboratory tools: for reaction and kinetic studies relies highly on the capability of performing online analyses, which opens great opportunities for the integration of spectroscopic detection techniques. This paper gives an overviews of the State-of,the-art in the combination of microreactors with spectroscopic analyses for Online reaction monitoring and catalyst characterization. In this upcoming field, many studies have been carried out combining fluorescence, ultraviolet visible, infrared, Raman, X-ray, and nuclear magnetic resonance spectroscopy. Current research progress is reviewed, with emphasis on the existing integration schemes and selected application examples that demonstrate the potential of online spectroscopic detection for rapid microreactor process analysis and optimization. An outlook on the future development in this area is also presented.