In many catchments, there are groundwater bodies that are important for water supply and the maintenance of low flows in rivers. The proper management of groundwater is therefore an important topic in hydrology and all hydrologists need to understand the basic principles that govern the analysis and prediction of groundwater. Some of the necessary theory has already been covered in Chapter 5 where the basic principles, parameters and terminology were introduced in considering the monitoring of soils and ground waters. Here we will look at how the way in which groundwater bodies might react to changes in recharge or pumping conditions can be predicted. In doing so, we will recognise that such predictions are now commonly made using modelling programs such as MODFLOW and ASM (see Section 15.5). In applying such models, however, it is important to have a proper understanding of the concepts and assumptions on which they are based.