Modelling is one of the central elements in a quantitative study. The ability to mathematically represent relationships between variables of interest (say, contaminants) allows us to test hypotheses using data. This course builds upon what you have learned in quantitative reasoning foundation viz modelling. We look at modelling within the context of pollution control engineering (specifically water pollution), and see how it allows us to describe complex systems and processes. Why does a system behave the way it does? Are we able to predict how it will evolve if left undisturbed? Can we determine how a system will respond to a change? The course is broken into three parts: the first two parts delve into material balance models and runoff estimation models, respectively. The last part looks at monitoring and sampling programmes that collect data for model validation, parameter estimation and other pollution control objectives. We begin by covering some basic concepts of environmental chemistry. We will then look at how material balance models can be used to better understand organic and groundwater pollution. Runoff estimation models are introduced through their application in flood control, an issue close to our hearts here in Singapore. Finally, we examine what goes into planning a pollution control monitoring programme, and review some case studies in sampling strategies.