A review is given on the past and current developments for in situ observation of membrane processes. Optical and non-optical probes for the membrane, fluid boundary layer, and bulk fluid have expanded enormously since the 1970s. Optical techniques include direct observation through membrane and related techniques, refractometric and interferometric techniques, photo-interrupt sensors, video, fluorescence, and particle image velocimetry. Non-optical techniques include impedance spectroscopy, ultrasonic reflectometry, radio isotope technique, NMR, SANS, computer aided tomography, electrochemical methods, and constant temperature anemometry. The scope for these techniques and technical challenges are discussed. (C) 2004 Elsevier B.V. All rights reserved.