Mobility is a concept frequently used in soil science to estimate the risk of contamination of other environmental compartments. However some confusion exists over the definition of mobility, originating from the fact that this term is used in different fields. Consequently many operational parameters have appeared in relation to mobility estimation. This leads to great difficulties in comparatively interpreting practical data obtained under different conditions, and especially the biological significance of mobility. A standardization of the definition of mobility, taking into account the time scale, should be recommended.