This booklet is a supplement to the WHO monograph Tools for assessing the O&M status of water supply and sanitation in developing countries which comprises nine tools for measuring and evaluating the effectiveness of operations and maintenance (O&M) of water supply and sanitation services. The WHO tools are: Tool 1: Effectiveness of the O&M management system. Tool 2: Guidelines for an audit of O&M. Tool 3: A framework for assessing the status of O&M. Tool 4: Guidelines on O&M performance evaluation. Tool 5: Guidelines on O&M performance reporting. Tool 6: Guidelines for the selection of performance indicators. Tool 7: Performance indicators for water supply and sanitation. Tool 8: Potential information sources. Tool 9: Participatory information-gathering. Tool 7 suggests performance indicators which are specific to water supply and sanitation; all other tools are generic and apply equally to any other of the urban services. Tool 7A has been produced as a supplement to Tool 7 which provides some indicators in relation to these other urban services. On inspection of Tools 7 and 7A, however, it can be seen that it is relatively straightforward to develop equivalent performance indicators oneself for one's own use. An additional Tool 10 has also been prepared to offer advice on indicators for technical, financial and institutional sustainability.