The application describes techniques for monitoring the rate of flow of fluid in a conduit (205) using fiber optic distributed acoustic sensing which are especially applicable to flow in oil and gas wells. The techniques also allow for calibration of the position of the channels of a fiber optic distributed acoustic sensor arranged along a fluid carrying conduit. The techniques comprise introducing a first acoustic stimulus (303) travelling in one direction and a second acoustic stimulus (304) travelling in the opposite direction. Flow of the fluid (v) results in a difference in the propagation velocity of the two acoustic stimuli. The first and second acoustic stimuli create a standing wave and the beat effect due to the flow induced wavelength variation is detected and used to determine flow rate. The acoustic stimulus may be introduced to travel through the conduit to provide the first stimulus and to reflect from the end of the conduit and travel back as the second acoustic stimulus.