Ferropnictides at high magnetic fields: the role of pairing symmetry and impurity scattering

An overview of recent results on the effect of impurity scattering and pairing symmetry on the upper critical field $H_{c2}(T)$ and critical temperature of oxypnictides at very high magnetic fields is given. The role of multiband effects and different scattering impurity channels on the observed anomalous temperature dependencies of anisotropic $H_{c2}(T)$ significantly exceeding the BCS paramagnetic limit is addressed.