Due in part to the high energy of the LHC, half of the top quark pair events are produced with additional high-momentum jets, which usually arise from radiation processes calculable in high-order perturbative QCD. These additional jets cannot be unambiguously associated with the top or initial-state quark from which they originate. This leads to measurement uncertainties that often limit the measurement precision of many quantities of interest, for example, of the top quark mass. Moreover, events with top quark pairs and additional jets give the largest contribution to the background plaguing the measurement of ttH. While predictions of the inclusive top pair production cross sections have reached very high precision, the prediction of additional QCD radiation is difficult and significantly less precise. Several theoretical approaches exist that show significant differences in their predictions of the QCD radiation. The ATLAS DESY group has measured the additional jet production in top events at 7 TeV and at 13 TeV  as shown in this plot.