A system and method for continuous, real-time process monitoring and control by means of near-infrared (NIR) spectroscopy provides analysis of static or flowing fluid streams which may range from clear to highly optically dense, including fluids primarily of aqueous composition. A NIR source passes through a wavelength selector to select one or more spectral segments, which are passed through the fluid stream at a fluid sampling interface and received at a sensor. A wavelength reference material is positioned in the optical path for calibration. Quantification of a plurality of characteristics or parameters of a fluid and suspended solids or cells contained therein may be performed. An all-solid-state implementation of the optical system ensures high robustness in laboratory and industrial settings.