Water quality models are widely used across contaminated sites, mining, and environmental management, yet they are often perceived as complex “black boxes.” This presentation provides a practical, accessible overview of water quality modelling, focusing on how models are developed, what they represent, and how they can be used to support environmental decision-making. We begin by linking water quality models to the conceptual site model, emphasizing that models are structured representations of site understanding rather than standalone tools. The talk then outlines the types of data and disciplines that inform modelling, including hydrogeology, geochemistry, hydrology, and biogeochemistry, and illustrates how these inputs interact to control contaminant fate and transport. Different modelling approaches—from simple screening tools to reactive transport models—are compared, with guidance on selecting fit-for-purpose methods depending on project stage and regulatory needs. We also discuss the role of multi-model approaches in addressing uncertainty and improving confidence in predictions. Finally, the presentation provides practical guidance on interpreting and evaluating model outputs, with a focus on understanding assumptions, uncertainty, and decision relevance. This talk is designed for environmental professionals seeking to better understand and apply water quality modelling in their work.