The Underlying Event in Hadronic Collisions

I review studies of the underlying event (UE) in hadronic collisions, dating from the first CDF studies in 2000 to the latest LHC findings and surprises. I explain the CDF quantum chromodynamics (QCD) Monte Carlo model tunes and describe how well the Tevatron tunes did at predicting the behavior of the UE at the LHC. In a very short time, the LHC experiments collected a large amount of data at 900 GeV and 7 TeV that can be used to study the UE in great detail. I review the LHC UE results and compare them with one another, with the Tevatron results, and with some of the LHC QCD Monte Carlo model tunes. I also explain the relationship between minimum bias collisions and the UE and discuss new techniques for studying the UE (i.e., techniques beyond the traditional approach).