A good understanding of the filtering and cleaning capacity of the soil is of paramount importance for assessing the vulnerability of the underlying aquifers. This understanding is a must to better define policies for land use, the use of chemicals and the dumping of wastes. An international workshop was organized at the Catholic University Leuven, Belgium in November 1999 with the objective to present the state-of-the-art on: the physical and chemical aspects of water and solute transport in soils; scale dependence of soil physical and chemical properties and processes; up-scaling of soil information; preferred pathways for water and solute flow; parameter identification; soil-rhizosphere interactions; model calibration, validation and application; and assessment of uncertainty in model predictions. These proceedings address new process descriptions, methodologies and techniques for the characterization of model parameters, applicability of simulation models through case studies on calibration and reliability assessment, and uncertainty aspects. The knowledge on transport processes in porous media were presented for problems at local, field and regional scales.