When the LHC beams cross in ATLAS, each crossing has on average over 20 hadronic interactions. They are mostly low-pT and "uninteresting" events. However, they will necessarily be superimposed on the high-pT and "interesting" events which we normally simulate. Pile-up refers to the mechanism of adding these additional events to the high-pT one in order to simulate what we actually observe. We have ~2 people from SLAC who are developing tools to validate that it is done correctly.