Simulations of events taking place at the LHC play key role in all experimental analyses. Starting from the basics concepts of QCD, we first review how accurate predictions can be obtained via fixed-order calculations at higher orders. Parton showers and event generation are then introduced as a means to achieve fully exclusive predictions. Finally the recent merging and matching techniques between fixed-order and fully exclusive simulations are presented, as well as their implementations via the MLM/CKKW and MC@NLO/POWHEG methods.