Studies undertaken in Belgium, France, Germany and Scotland have shown that the deposit situated at the water-sediment interface (termed the Near Bed Solids) is possibly the main source of pollutants for Combined Sewer Overflows. However, this interface has so far been identified with different experimental methods and the results are not concordant. A standard protocol for the examination of the interface material, using an endoscope, allows visualisation of the deposits in a non-destructive manner at the water-sediment interface. This has been used in Le Marais catchment and also Dundee. Based on the experimental data collected thus far, a number of models have been proposed to estimate NBS and predict pollutant loads of consequence for CSOs. New combined approaches have to be developed and more data (such as Oxygen Utilisation Rate and pollutant release) are required to take properly into account the role of the NBS in CSO spill pollution.