Multi-phase flow is an important phenomenon in process engineering. World-wide, large efforts are undertaken to extend the physical understanding of the processes in multi-phase flows. The mixture of a gaseous and a liquid phase is a special kind of a multi-phase flow. The present lecture reports about a wire-mesh sensor for the fast measurement of transient spatial distributions of gas and liquid fractions in this type of a two-phase flow. It can also be used for a fast visualization of the flow structure with a rate of up to 10,000 frames per second. Another field of application is the concentration measurement of dissolved components in single-phase liquid flow.