On small rivers it is often convenient to measure flow by means of a weir or flume. Such structures have the advantage that they are less sensitive to the downstream conditions, the channel roughness and the influence of backwater than the velocity-area method for example. A good example of the use of measuring structures is to be found in the UK where, mainly because of the small rivers and the influence of backwater, the hydrometric network of some 1400 streamflow stations contains about 750 measuring structures. Practically all of these are of a type which has been precalibrated in the laboratory, some of which have been check-calibrated in the field. In this chapter, those weirs and flumes which have been precalibrated and standardised by the International Organization for Standardization or the British Standards Institution will be discussed with an additional section on non-standard weirs.