This booklet outlines the properties of geological materials which enable them to accept, store and transmit groundwater, together with a description of the principal types of aquifer which are commonly found to occur in the field. Sources of groundwater are described in order to provide an understanding of the hydrogeological modelling exercise. The governing equations for steady state and non-steady state (transient) groundwater flow, are presented together with a brief overview of a range of modelling techniques, including both analytical and numerical models, the use of diffuse recharge as a model calibration tool and recent developments in the field of inverse modelling are also discussed.