Powered TwoWheelers are a highly vulnerable group of road users, as every year more than three times as many PTW riders as car passengers are killed in traffic accidents with injury to persons. In addition to the official statistics these kinds of accidents are also recorded in so called In-Depth accident databases. The analysis of the circumstances of an accident as well as the investigation of the most probable accident causes contribute to traffic safety to a great extent. To be able to compare the results of analyses of different (In-Depth) accident databases and the measures that derive from these results, it is necessary to be able to compare the variables defined in each of the different databases. Therefore, the main goal of this thesis is to harmonise the accident causes and the accident causation factors defined in the accident databases Unfalldatenmanagement UDM, Initiative for the global harmonisation of accident data IGLAD and Motorcycle Accidents In-Depth Study MAIDS. The challenges of this process as well as the possibly resulting restrictions are carefully documented. In addition to that the suitability of the examined databases, specially the one to record traffic accidents including PTW, is evaluated. Firstly, the chosen databases are described in general and analysed for their specific ability to record PTW accidents. Secondly, an approach to harmonise the main accident causes as well as the main accident causation factors defined in the respective databases is made based on theoretical considerations. Thirdly, this attempt of a harmonisation is tested on its validity by using data collected from real PTW accidents in the German state of Hessen. Finally, some recommendations for possible extensions of the analysed databases in order to record PTW accidents more accurately are given, based on the results of the preceding harmonisation and the experiences gained during the process. The achievements of this thesis are, on the one hand, the harmonisation of the main accident causes as well as the accident causation factors using selected accident databases and the testing of this harmonisation and, on the other hand, some general recommendations for extending such databases to more accurately record PTW accidents.