Droplets in sprays and aerosols play an important role in the environment and industry, for example in our atmosphere, in combustion engines, in spray dryers and in pesticide management. In all of these areas, a detailed knowledge of the properties of single droplets is essential to improve our understanding and modelling of them. On the other hand, optical measurement techniques are necessary for diagnostics in sprays and bulk aerosol. To both aspects this dissertation contributes important new results. In chapter 2 a new geometry of electrodynamic trap is shown, consisting of four electrodes in the form of tubes. This trap provides a large optical access of 360 degrees in the horizontal plane and 36 degrees in the vertical plane allowing a flexible combination of optical measurement techniques and the observation of light scattering over a large angular range. The new trap yields greater optical access than other existing geometries with the exception of ring electrodes. In comparison to the latter, under similar conditions, the new electrodes yield improved stability, which is necessary to enable trapping of droplets in a gas stream. A variety of optical measurement techniques successfully combined with the new trap are presented in chapter 3. Thereby the size, refractive index, concentration and the density of droplets, and also drag forces and gas speeds are determined. Through contact free measurements any disturbance of heat and mass transfer and droplet shape by wall contacts are avoided. The following chapters present new optical measurement techniques and new applications in the electrodynamic trap: For the first time, the disappearance of morphology dependent resonances in water droplets of 50 micrometers diameter under the illumination of femtosecond laser pulses is shown experimentally in chapter 4. This contributes to the development of a new intensity based measurement technique for sprays. Chapter 5 shows how single water droplets of 50 micrometers diameter with a spherical air inclusion are trapped for the first time. Their angular dependent light scattering is recorded for the first time to provide experimental data for the verification of existing theory. The large optical access of the new trap allowed scattered light to be collected over a range of more than 120 degrees. Moreover, the time dependent light scattering is used to successfully determine the diameter of the droplet and the inclusion as well as the position of the inclusion with a precision of +/- 1.5 micrometers. Observed glare points have been found to be in good agreement with predictions. In chapter 6 the measurement of mass transfer (Sherwood number) of spherical droplets for Reynolds numbers near one and Schmidt number 0.64 is presented. The current results build on previous works thanks to improved precision of the Sherwood number and the large number of 700 investigated droplets. These results will help to develop models in the aforementioned applications. Chapter 7 shows how the same measurement is combined with temperature measurements on single water droplets using spontaneous Raman scattering. Among investigations into the influence of a gas stream on the evaporation of single droplets this work is the first with a contact free temperature measurement. Within the measurement uncertainty of the temperature of +/- 4K, the results justify temperature calculations used in the previous chapter. These experiments show the new electrodynamic trap to be a powerful tool for the investigation of single droplets. This can lead to further important innovations for industry, agriculture and environment.