Pharmaceuticals are environmental contaminants causing steadily increasing concern due to their high usage, ubiquitous distribution in the aquatic environment, and potential to exert adverse effects on the ecosystems. After being discharged from wastewater treatment plants (WWTPs), pharmaceuticals can undergo transformation processes in surface waters, of which microbial degradation in river sediments is considered highly significant. In spite of a substantial number of studies on the occurrence of pharmaceuticals in aquatic systems, a comprehensive understanding of their environmental fate is still limited. First of all, very few consistent datasets from lab-based experiments to field studies exist to allow for a straightforward comparison of observations. Secondly, data on the identity and occurrence of transformation products (TPs) is insufficient and the relation of the behavior of TPs to that of their parent compounds (PCs) is poorly understood. In this thesis, these knowledge gaps were addressed by integrating the TP identification using suspect/non-target screening approaches and PC/TP fate determination. The overarching objective was to improve the understanding of the fate of pharmaceuticals in rivers, with a specific focus on water-sediment interactions, and formation and behavior of TPs. In paper I, 11 pharmaceutical TPs were identified in water-sediment incubation experiments using non-target screening. Bench-scale flume experiments were conducted in paper II to simultaneously investigate the behavior of PCs and TPs in both water and sediment compartments under more complex and realistic hydraulic conditions. The results illustrate that water-sediment interactions play a significant role for efficient attenuation of PCs, and demonstrate that TPs are formed in sediment and released back to surface water. In paper III the environmental behavior of PCs along stretches of four wastewater-impacted rivers was related to that of their TPs. The attenuation of PCs is highly compound and site specific. The highest attenuation rates of the PCs were observed in the river with the most efficient river water-pore water exchange. This research also indicates that WWTPs can be a major source of TPs to the receiving waters. In paper IV, suspect screening with a case-control concept was applied on water samples collected at both ends of the river stretches, which led to the identification of several key TPs formed along the stretches. The process-oriented strategies applied in this thesis provide a basis for prioritizing and identifying the critical PCs and TPs with respect to environmental relevance in future fate studies. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.</p>