The negative effects on building projects due to unforeseeable events, such as unusual weather factors or other occurrences, which were unforeseeable at the time of the conclusion of the agreement, are not rare incidents in the building business anymore. The causes for such disturbances can be manyfold. In the case of unforeseeable events, the consequences may not only be a delay in construction, but unusual weather conditions, for example, can lead to enormous damages, for which nobody feels responsible. All of these aspects will be dealt with in this thesis, and the statutory regulations according to the ABGB, the contractual regulations of the ÖNORM B 2110, the ÖNORM B 2118 and the RVS 10.01.11 will be compared in order to state the various legal consequences. The first part of this thesis will explain the term 'unforeseeable events' and give several examples. Subsequently, the building contract, which constitutes the basis of the entire building business, will be elaborated on. The core of the thesis consists of the ÖNORM B 2110. In this context, the different ways of regulation of the assignment of spheres will be examined. Furthermore, will be dealt with divergence of performance and its legal consequences, as well as the various possibilities of right to rescind the contract and delay. All these subjects will be regarded from the legal as well as from the contractual perspective. Subsequently, the essential changes of ÖNORM B 2118 and RVS 10.01.11 concerning unforeseeable events will be explained. After that the possibility of insurances in the building business will be examined. In this context, the question is whether the conclusion of an adequate contract of insurance would be the solution to the problem of unforeseeable events. Finally, two case studies will illustrate what and when exactly an event is considered unforeseeable.