The whole system of water quality monitoring is aimed at the generation of reliable data, i.e. data that accurately reflect the actual status of the variables which influence water quality. It is acknowledged that simply generating good data is not enough to meet objectives. The data must be processed and presented in a manner that aids understanding of the spatial and temporal patterns in water quality, taking into consideration the natural processes and characteristics of a water body, and that allows the impact of human activities to be understood and the consequences of management action to be predicted. This is not to say that water quality information must be presented in a way which requires the user to appreciate the lull complexity of aquatic systems. The information should provide the user with the understanding necessary to meet the objectives behind the monitoring programme. The importance of setting objectives in the design and implementation of monitoring programmes is discussed in Chapter 3. Objectives imply that the activity has a purpose that is external to the monitoring system; e.g. a management, environmental policy, public health or research purpose. The intent is to use the information to explain water quality, to communicate the information more widely, or to control water quality. Consequently, there is little point in undertaking a monitoring programme unless the resultant data are to be used in fulfilling the objectives.