Bioaccumulation is defined as the enrichment of a compound in an organism relative to the surrounding water or its food, and is an important endpoint in chemical risk assessment. Under laboratory conditions, bioaccumulation is measured as bioconcentration factor (BCF) or biomagnification factor (BMF) in fish, which represents compound exposure via the respiratory (water) and dietary (food) pathway, respectively. Such tests are resource intense, costly and ethically questionable, due to the sacrifice of large numbers of fish. A potential in vitro alternative for the bioaccumulation assessment with fish are permanent fish cell lines. Cell tests have the advantage that the cells conserve many important processes for bioaccumulation in fish. Most prominent fish cell lines are the rainbow trout (Oncorhynchus mykiss) gill and liver-derived cell lines, RTL-W1 and RTgill-W1, which have successfully been used to predict the BCF of a neutral model compound in fish. However, application of fish cell lines for bioconcentration predictions has not yet been assessed for ionizable organic compounds (IOC). IOC are frequently detected in natural water bodies and aquatic organisms, yet, their assessment for environmental hazard assessment is not well covered in international regulation to date. Motivated to fill this knowledge gap, this thesis set out to assess the bioconcentration potential of IOC in RTL-W1 and RTgill-W1 cells, and predict IOC bioconcentration in fish. For this purpose, three cationic IOC and four anionic IOC, which are ionized at biologically relevant pH, were selected. Firstly, a method was developed which enabled the derivation of full mass balances of the tested IOC in the experimental set up with fish cell cultures. Bioconcentration assays were then conducted with non-toxic exposure concentrations, which were previously determined in acute cytotoxicity assays with RTgill-W1 cells. From measured compound concentrations in the medium and cells, in vitro BCF were determined and compared to in vivo BCF and BMF. In combination with in vitro biotransformation assays and partition coefficients to biological matrices, the derived in vitro BCF may serve as a line of evidence for conservative estimates of the bioconcentration potential of IOC in fish. This highlights the potential of this method as a future screening tool for environmental risk assessment. The pH-dependent octanol-water partitioning and membrane lipid-water partitioning best predicted the measured in vitro BCF for anionic and cationic IOC, respectively. The experimental mass balances and in vitro BCF were compared to different model approaches, to gain insights into the relevant cellular accumulation mechanisms and to evaluate the applicability of a novel screening approach that uses the human volume of distribution, VD and the blood water partition coefficient, KBW. The model outcomes were that the test compounds' affinity to membrane lipid and protein are most influential for compound accumulation. The prediction of bioconcentration in fish and cells using human VD and KBW showed to be a promising read across to screen the bioconcentration potential of IOC in fish, where availability of reliable KBW remains the main challenge. This thesis highlights key areas to aid in the efforts towards the replacement of animal experimentation in bioaccumulation assessments of organic compounds. Additionally, it emphasizes the value of in vitro assays for environmental risk assessment.