CityGML is a standardized data format used to store the semantic information and geometries of buildings and other object classes of 3D city models. The Level of Detail of current state of the art city models (LoD2) is not sufficient for accurate environmental simulations like noise, the solar potential of windows and other types of analyses. An LoD3 building model represents the full architectural exterior of a building with balconies, windows and so forth. The generation of these models needs to be automated as it is otherwise infeasible due to the required high amount of manual labour. In the architectural world, detailed building models are created in IFC format. This thesis shows that it is possible to automatically generate valid and semantically rich CityGML LoD3 building models directly from IFC models. Also an initial investigation is done on the possibilities for the conversion of IFC models to CityGML LoD4. For the conversion the semantic and geometric validity requirements are determined for CityGML. A methodology for the conversion is developed and a prototype implementation is made to prove the effectiveness of the conversion. The conversion consists of three parts: 1) The extraction and mapping of IFC semantics to CityGML semantics; 2) A geometric generalization which extracts the exterior shell using a transformation based on Boolean and morphological operations; 3) Semantic and geometric refinements which optimize the model for analyses. The developed prototype is able to successfully convert IFC models to CityGML LoD3. All the resulting models were geometrically validated according to the ISO19107 standard, and semantics were checked manually. Few improper semantics occur in the output due to missing semantics in IFC. For example, there are no semantics for balconies or dormers in IFC. Recommendations are given to improve the alignment between the two formats. For IFC additional semantics are recommended whereas it is important for CityGML to specify how certain aspects are to be modelled. The research presented in this thesis can be used as the foundation for future work on the interoperability between Architecture and Geomatics. The software package is open source and freely available at https://github.com/tudelft-gist/ifc2citygml.