In this master thesis numerical studies of the dynamic behaviour of natural circulation steam generators are presented. The main motivation of this thesis was to study the dynamic behaviour of natural circulation steam generators for different starting sequences. A special topic was to study the behaviour of the water circulation for the case that the length of the siphon of a heat recovery steam generator was reduced. In the first part the basic fundamentals of steam generators like the different building types or circulation systems are presented. The emphasis is on natural circulation systems. As a special application of the steam generators, heat recovery steam generators, which are used in combined cycle power plants, are introduced. In the second part a heat recovery steam generator is modelled and implemented in the software applications NOWA, DBS and APROS. After describing the preparation of the thermotechnical data, the final results of the different calculations are shown and discussed.