Tungsten and tungsten-based materials will be used in neuralgic sections of fusion reactors. A disadvantage of this choice of the material that has to be considered is the brittleness at “low” temperatures, below a few hundred centigrade. Both, a decrease of the brittle-to-ductile transition temperature and an increase of materials’ toughness at low temperatures will contribute to the successful application of tungsten for fusion applications. Hence, it is necessary to comprehensively understand the mechanisms governing the deformation of tungsten, in particular the fracture behavior. An increase in fracture toughness was observed when alloying tungsten with rhenium and through microstructural modification of tungsten-based materials. Rhenium is a rare element and it is the only element known to markedly increase the ductility and fracture toughness of tungsten. Specific microstructure design is then the best route to produce large batches of “tough” tungsten. For the work presented in this thesis, severe plastic deformation was used for production and deformation of tungsten based materials. Fracture experiments using ultra-fine grained composites of tungsten-vanadium and tungsten-tantalum show an increase in fracture toughness in two out of three possible testing directions. An increase in fracture toughness for all testing directions seems difficult, maybe even impossible, to realize by microstructural design. To better understand the separate influences of certain microstructural constituents such as grain boundaries, pores and precipitations on the fracture behavior, fracture experiments using micrometer-sized samples – notched bending beams – were developed and improved. A so-called “ion slicer”, typically used for transmission electron microscopy sample production, was used to improve the final sample production with a focused ion beam workstation. In order to identify and better understand the fracture processes and to develop the analysis of the micrometer–scaled experiments, single crystalline tungsten was chosen as a model material. Linear elastic fracture mechanics is not applicable due to the large plastic zone in relation to the small sample size; elastic – plastic fracture mechanics has to be used. In doing so, the fracture toughness of single crystalline tungsten was determined. Furthermore, an experimental basis was set in order to better understand crack propagation in tungsten.