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Knowledge of the movement of water and solutes through the vadose zone of soils is essential for reliable predictions of pollution risks to groundwater. So far, most models simulating water and solute transport through the unsaturated zone have assumed homogeneous infiltration and a subsequent downward movement of the wetting front parallel to the soil surface. This type of stable flow, however, is uncommon in field soils. Deviations are caused by a variety of mechanisms. Firstly, preferential flow of water and solutes may occur in well-structured clay and/or peat soils owing to the presence of shrinkage cracks. Secondly, preferential flow may also occur in non-structured sandy soils, owing to the development of unstable wetting fronts. Fingers or preferential flow paths occur if (1) the hydraulic conductivity increases with depth, as is encoutered in soils with a fine-textured layer covering a coarse-textured layer; and (2) the soil is water repellent. Irregular wetting patterns and preferential flow paths have been established in the Netherlands in wettable dune sands and in water repellent sand, clay, and peat soils.