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. While these background events could in principle be simulated, it is far simpler to use real data with a zero-bias trigger. It naturally includes all background sources in the right ratio. Overlay refers to the mechanism of adding these real-data background events to the high-pT one in order to simulate what we actually observe. We are developing tools to validate that it is done correctly.