The 16th World Congress of Soil Science, held in Montpellier in August 1998, emphasized the relationships between human societies and the soil cover. Among these relationships, Soil and Water Conservation is a well identified challenge, which has given rise to more than 50 years of research on soil erosion around the world. One of the main concerns is to allow sustainable soil cultivation. To address this issue, plot and field scale experimentation, measurements and modelling have enriched our knowledge of (i) the effects of factors in combination and (ii) the processes themselves. Building on this knowledge, significant effort has been done to implement hydrological and erosion models. With the development of GIS, there has been a great increase in distributed modelling at catchment scale: if we fully accept the uncertainties and failures as well as the successes, what have we learned? Processes at local and field scales strongly interact, both in space and time, to create distinctive patterns of runoff generation and erosion/deposition at catchment scale. For hillslopes and catchments, the failure of erosion modelling reveals the lack of knowledge about water flow pathways (infiltration/runoff, effects of critical discharge local thresholds...) and, more generally, the effect of soil cover and soil patterns on hydrology, together with how these processes vary with scale.
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