In complex environmental systems, we are rarely able to find analytical solutions to the problems we are trying to solve. This chapter introduces scaling methods for simplifying complex system into manageable models. For example, by defining a characteristic time-scale for each process one can quickly determine which processes are unimportant and can be discarded, greatly simplifying the problem. This chapter also looks at the distribution of residence times in real aquatic systems and how this impacts upon the chemistry of the system. The problems in this chapter require the practical application of time-scale analysis to an environmental engineering problem and an understanding of how the residence time distribution can affect the removal efficiency of a wetland.