The Manual on Stream Gauging (WMO No. 519) was first released in 1980. Since then, however, there have been significant advances both in the approach and the methodologies employed. Consequently, at its twelfth session (Geneva, October 2004), the WMO Commission for Hydrology (CHy) decided to meet the identified needs of the National Hydrological Services by revising the Manual to include the newer technologies that have been introduced over the period and are currently employed in this crucial field. In this context, Volume I of the Manual on Stream Gauging encompasses the topics of gauge height measurement, stream velocity and stream discharge, whilst Volume II focuses on the discharge rating relationship.