The main emphasis of the paper is on cross correlation flowmeters, which are developing to a stage where they can successfully solve industrial and environmental measurement problems, ranging from the flow of highly polluted liquids in pipes to the flow of gas from volcanic jets. Cross correlation flowmeters are based on measuring the transit time of a tagging signal (turbulence, clumps of particles, etc.) in the flow between two axially separated sensors. The transit time is measured by a cross correlator. The design of the various subsystems is discussed in some detail and the various sensing techniques required for specific applications are described. Cross correlation flowmeters have become a realistic proposition because of the reducing cost of large scale integrated circuits and microprocessors from which the correlator may be realised, a number of different designs of cross correlator are described and their relative merits discussed.