Poor surface water quality is still a significant problem in many parts of the world. It can often limit the use of this vital resource and in more extreme cases can harm human and other life. A basic need is to establish the nature, extent and magnitude of the problems. To gain a quantitative picture it is necessary to undertake detailed quality and status assessments. Such assessments establish the scope of environmental impacts and effects, and provide a quantitative baseline against which future quality can be compared and progress monitored. Quality assessments or status reviews would enable the key problems and issues to be defined, and their magnitude and importance quantified. Problems for priority action can then be identified. Once key problems have been identified there are mechanisms to enact improvements and changes. These include the setting of standards and targets for water and sediment quality, quite often for defined actual or potential uses of the waters, such as for example for potable water supply and for fisheries. Compliance with the standards and progression towards the targets has to be assessed generally through appropriate monitoring. Monitoring might include biological as well as chemical and physical measurements of quality, and these might also be expressed as some form of index of quality. Periodic assessments of general quality on a national or international basis are also used to monitor overall progress. Nationally this might entail the use of general classification schemes expressing quality as a combined index or score, or individually for different components of the aquatic system, e.g. chemical, biological, aesthetic, and sediment quality.