Challenges for QCD theory – some personal reflections –

At the LHC all processes are QCD ones, whether “signal” or “background”. In this review the frontiers of current QCD research are addressed, towards increased understanding, improved calculational precision, and role in potential future discoveries. Issues raised include the limits of perturbative QCD calculations and parton distribution usage, the nature of multiparton interactions, the impact of colour reconnection on physical observables, the need for progress on hadronization modelling, the improvements of parton showers and their combination with the matrixelement description, the use of QCD concepts in Beyond-the-Standard-Model scenarios, and the key position of event generators and other software in the successful exploration of LHC physics. On the way, several questions are posed, where further studies are needed. To appear in the proceedings of the Nobel Symposium on LHC results, Uppsala, Sweden, 13 – 17 May 2013