This thesis demonstrates that a scintillometer is able to obtain a path-averaged value of the crosswind (i.e., wind component perpendicular to a path). A scintillometer consists of a transmitter and a receiver spaced a few hundred meters to a few kilometres apart. The transmitter emits light with a certain wavelength, which is refracted by scintillations in the atmosphere (eddies with a different temperature and moisture content than their surrounding). The atmosphere is turbulent, thus the receiver of the scintillometer measures the intensity of the fluctuations of the light. Two scintillometer setup next to each other in principle measure the same eddy field except for a time-shift. It is known that this time-shift is linked to the crosswind: the lower the time shift the stronger the crosswind. This thesis shows that experimental calibration in the field to measure the crosswind with a scintillometer is not necessary. Also we developed two new algorithms, which are able to obtain the crosswind from the scintillometer signal. First, the algorithms were validated with measurements made above a flat grassland field. Later, measurements took place over more complex terrains (i) above the city of Helsinki, Finland and (ii) next to a runway at Schiphol airport, the Netherlands. We highlight that even in these complex terrains the scintillometer was able to obtain the crosswind correctly. At Schiphol airport also other applications of scintillometry were investigated: visibility measurements, and wake vortex detection. To use scintillometers as a visibility sensor, more research is necessary. In contrast, the scintillometer proved to be able to detect wake vortices created by airplanes during the night, when the atmospheric turbulence is low.