Molybdenum is a refractory metal and widely used in the lighting-, electronic-, coating-, and high temperature industry. The combination of a high electrical and thermal conductivity, together with a low coefficient of thermal expansion and a high strength at elevated temperature, makes molybdenum perfectly suited for high performance products. However, a brittle-to-ductile transition at temperatures close to room temperature as well as an increased embrittlement of the recrystallized state complicate the production and the use as a structural material. In molybdenum and its alloys it is believed that grain boundary segregation plays a significant role regarding the ductility and strength. Therefore, combining mechanical properties with the grain boundary chemistry on the nanometer scale is a promising approach to understand the embrittlement of molybdenum. In this PhD thesis high-resolution characterization techniques such as the three-dimensional atom probe were applied to investigate the grain boundary chemistry of powder-metallurgically processed technically pure molybdenum and its alloys. In order to study the solute decorations effectively a new atom probe sample preparation method with correlative transmission Kikuchi diffraction was developed. With this technique it was possible to characterize several high-angle grain boundaries of pure molybdenum and its alloys in order to combine crystallographic with chemical information of analyzed grain boundaries. The results of atom probe investigations on technically pure molybdenum reveal that oxygen, nitrogen and phosphorus are typical grain boundary segregation elements. These solutes are known to have a detrimental effect on the grain boundary strength and may provoke intergranular fracture. Mechanical tests support these assumptions, as they show that delamination cracks indeed mainly follow high-angle grain boundaries, which seem to be particularly weakened by the before-mentioned segregation elements. The present results further indicate that grain boundary segregation of carbon and boron has a beneficial effect in technically pure molybdenum, as these elements promote transgranular fracture and can lead to grain boundary strengthening. Additionally, concentration of detrimental segregation elements at grain boundaries can be reduced by decreasing the grain size or by reducing the overall content of these solutes. The present work clearly identifies the effect of typical grain boundary segregation elements on the strength and fracture behavior of technically pure molybdenum. This newly gained knowledge will help to develop strategies to further enhance the material’s mechanical properties, especially in respect to its brittleness, and paves the way for a targeted grain boundary engineering.