The proliferation of non-linear and time-variant loads is causing a number of disturbances on the electric network, from a more and more significant distortion of both currents and voltages, to transient disturbances on the supply voltage. In this respect the electric network behaves as an "healthy carrier" of disturbances, so that a disturbance generated by one customer can be distributed to other customers, causing possible damage to their equipment. The measurement of the quality of the electric power in a network section is therefore becoming an impelling need, especially in a deregulated electricity market, where each actor can be responsible for the injection of disturbances. However, there are still some respects of power quality measurement, from both the methodological and instrumental point of views, that are still unsolved and require to be carefully analyzed. The paper gives a survey of these problems and some indications about the present trends of the research work in this field.