This chapter describes the historical development and present applications of water-quality models for organic chemical compounds (e.g., polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs)). Various types of water-quality models are described, varying in the amount of compartments, in spatial dimension, and in time. Special attention is given to the processes acting on these chemical compounds (e.g., volatilization and decomposition) and to the quality and precision of the value of parameters in these processes. An overview is given of the most important water-quality models in use now.The chapter concludes with attention and discussion points on accuracy and precision of model predictions and how to use these models for hazard and risk assessment. At the end, recommendations are presented for improving the further development and application of water-quality models by benchmarking and monitoring.