This publication is based on the findings of a coordinated research project (CRP) and presents the results obtained from ten field sites, located under different climatic conditions, equipped with instrumentation to monitor and sample water and solute transport through the unsaturated zone. A number of isotope and hydrochemical tools have been used to study groundwater recharge and transport of pollutants in this zone. The information gained from these studies has led to a better understanding of flow and transport processes which is essential to assess the vulnerability of groundwater to contamination.