MEMS based devices have a huge potential in different monitoring applications ranging from environmental control to medicine. This paper shows principles and advantages of two miniaturized NIR/IR-spectrometers, based on different MEMS technologies. Small size, portability and cost efficiency as well as high accuracy and mechanical robustness make both spectrometers suitable for use in such applications. The paper depicts principles and advantages of both devices. Performance parameters and a selected application, the monitoring of biogas plants to raise their efficiency, are presented.