Two-phase flows are of great relevance both in everyday life and in many technical processes. Condensed droplets in the air, for example, cause poor visibility as fog. In steam turbines, droplets form when the expansion of the steam extends beyond the dew point line into the two-phase region. To determine the exact thermodynamic state of the steam, the mass of water contained in the droplets must be known. Depending on the size of the droplets, they can be deposited on the flow leading components and subsequently cause damage to the steam turbine. Determination of the droplet size is therefore desirable as well. A satisfactory size determination of the water droplets can only be guaranteed by optical measuring methods. On this basis, a measuring system is developed in this thesis which captures the scattered light from illuminated droplets and enables a statement to be made about the droplet size. To introduce the topic, two-phase flows in general and the possibilities of their measurement are presented. This is followed by a detailed consideration of two-phase flow in low-pressure steam turbines and the measurement methods available for measuring wet steam. Some measuring systems developed in the past at RWTH Aachen University and other research institutions are discussed in more detail. Subsequently, the development of the measurement probe, which is the most important component of the measurement system, is discussed in detail. Based on the boundary conditions prevailing in the steam turbine, requirements for the measuring system are defined, which must be taken into account in the design of the various components. The most advanced manufacturing processes make it possible to develop a probe that meets almost all requirements. This is followed by the description of the measuring system and the necessary preparations to be able to carry out and evaluate measurements in the steam turbine. This includes the calibration of the measurement system as well as the procedure for the evaluation of the recorded data. Finally, the application of the developed measurement system in a 12.5 MW test steam turbine is presented. The dependence of the measurement results on the probe positioning, which had already been suspected in advance, is shown. In addition, the influence of the lacing wire installed in the turbine blade row on the droplet size can be demonstrated and quantified. The measured droplet sizes agree with other measurement results already published in the literature.