The scattering of massive electroweak vector bosons allows for sensitive tests of the mechanism of the electroweak symmetry breaking. In run 2 of the LHC several of these processes have been observed, and measurements with higher precision and for polarised bosons will happen in future runs. In this talk I review the status of theoretical predictions for vector-boson scattering processes. I discuss some details of the calculations and a selection of results. Moreover, the origin of large electroweak and QCD corrections is explained.