Event generators are used extensively in many LHC searches and therefore improving their theoretical accuracy is important for fully exploiting the LHC potential. I will briefly explain the basic concepts and approximations upon which these tools are built and review the theoretical developments that in the last decade allowed to improve substantially the accuracy of these programs. I will then explain how a combination of two of these methods (POWHEG and MiNLO) allows to match parton showers with NNLO QCD computations, and show results for Higgs and Drell-Yan and production.

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