MC models of multiple partonic scattering inevitably introduce many free
parameters, either fundamental to the models or from their integration with MC
treatments of primary-scattering evolution. This non-perturbative and
non-factorisable physics in particular cannot currently be constrained from
theoretical principles, and hence parameter optimisation against experimental
data is required. This process is commonly referred to as MC tuning. We
summarise the principles, problems and history of MC tuning, and the
still-evolving modern approach to both model optimisation and estimation of
modelling uncertainties.Comment: Contributed chapter to "Multiple Parton Interactions at the LHC",
World Scientific 201
