This publication is a compilation of the results obtained under the Coordinated Research Project (CRP) on Isotopic Composition of Precipitation in the Mediterranean Basin in Relation to Air Circulation Patterns and Climate, from 2000 to 2004. The isotopic composition of precipitation is closely connected with the conditions for rain formation, i.e. with the temperature of formation, the origin of the air masses, and the degree and mechanism of rainout. Stable isotopes are, therefore, powerful tools for investigating the precipitation and formation conditions, and for monitoring their changes in parallel to the observed climatic changes. Hydrologists, hydrogeologists and meteorologists were involved in the project and collaborated at the national and regional levels.