For a task like 3D building reconstruction, there are three main data sources carrying information which is reburied for a highly automated data acquisition. These data sources are aerial images, digital surface models (DSM), which can either be derived by stereo matching from aerial images or be directly measured by scanning laser systems, and -- at least for highly developed countries -- existing (2D) GIS information on the ground plan or usage of buildings. The way these different data sources should be utilized by a process of 3D building reconstruction depends on the distinctive characteristics of the different, partly complementary type of information they contain. Image data contains much information, but just this complexity causes enormous problems for the automatic interpretation of this data type. The GIS as a secondary data source provides information on the 2D shape, i.e. the ground plan of a building, which is very reliable, although information on the third dimension is missing and therefore has to be provided by other data sources. As the information of a DSM is restricted to surface geometry, the interpretation of this kind of data is easier compared to the interpretation of image data. Nevertheless, due to insufficient spatial resolution or quality of the DSM, optimal results can only be achieved by the combination of all data sources. Within this paper two approaches aiming on the combination of aerial images, digital surface models and existing ground plans for the reconstruction of three- dimensional building reconstructions are demonstrated.