Operational hydrology requires reliable and accurate instruments and methods of observation. It is useful to review developments, particularly those of the last decade. Two milestones of this development may be mentioned, both actively supported by the IAHS International Commission on Surface Water, namely the IAHS/UNESCO/WMO (1970) Symposium in Koblenz, and the WMO Seminar in Padova 1975 (WMO, 1975). In assessing the trends, the interconnections of hydrometry with network design and data processing are discussed. Suggestions are also submitted for further developments including research and the potential applications in one particular field, where new instruments and methods are badly needed, specifically the measurement of three-dimensional turbulent flow in streams and lakes. Problems and results are introduced in connection with three-dimensional flow measurement to give an example for the trend of development.