Contaminated groundwater poses a serious threat to drinking water resources all over the world. Even though contaminated water might be detected in observation wells, a proper clean-up is often only successful if the source of the contamination is detected and subsequently removed, contained or remediated. However, it is possible to significantly reduce the high costs of groundwater remediation when a focus is placed on source zone detection from the outset of a clean-up project.ModBack is a software that combines several existing modelling tools into one easy to use ESRI ArcGIS 10-based interface, helping to delineate potential contaminant source zones in the subsurface. This software is written in Visual Basic 3.5 and uses the ArcObjects library to implement all required GIS applications. It can run without modification on any Microsoft Windows based PC with sufficient RAM and at least Microsoft .NET Framework 3.5. Using ModBack requires additional installation of the following software: