Electromagnetic (EM) telemetry is a Measurement While Drilling (MWD) telemetry system used for transmission of real-time data during drilling. The thesis analyses the data gathered from the introduction of EM telemetry in continental Europe, particularly Austria and the Netherlands. Compared to the widely used mud pulse telemetry it does not require a drilling fluid for operation. The battery-powered system emits an ultra low frequency signal through the formation, which is received on the surface. Since the system does not depend on a continuous liquid column it allows constant communication, especially during underbalanced drilling, and helps to reduce circulation problems. During conventional overbalanced drilling the system allows to diminish non-productive time due to directional surveys as the data is received while making a connection. The main limitation of EM telemetry is how deep it can be used to still be able to detect a working signal. The attenuation of the electromagnetic field greatly depends on the resistivity of the overburden formations, which act as transfer media. This problem can be overcome by optimizing the transmission frequency and by adding a repeater to the drill string. Proprietary software is used to model and predict signal attenuation based on resistivity data from offset wells. These models are then compared to the actual recorded signal behavior. It is described how the measured signal is processed to make a systematic analysis possible. Further a method is shown how to calibrate the offset model to reflect the actual trend. This can be done in real time, while the system is deployed, to better predict the limitations based on the current operational parameters.