From a multiphase fraction meter for well mixed flow previously developed by Christian Michelsen Research AS (CMR), a multiphase flowrate meter has now been developed. The new flowrate meter is of a non-intrusive design, and mounted in a vertical upwards flow, it will accept flow regimes from dispersed bubble flow to churn flow without any pre-conditioning of the flow. Further development in order to extend the range to cover also slug flow is in progress. To measure flowrates non-intrusively without any mixing device, a signal interpretation model has been developed as a vital part of the system. The signal analysis takes into account knowledge of multiphase flow behavior. Such a knowledge is essential when the complex problem of multiphase flow metering is addressed. This paper therefore provides some basic background theory of vertical upwards gas-liquid flow, and thereafter discusses some important consequences of different flow conditions to a multiphase flow meter. Finally, the concept of the CMR multiphase flowrate meter is presented, together with test results from extensive laboratory testing of this new multiphase flowrate meter. 8 refs., 14 figs.