The dynamics of defect migration in molecular systems has attracted widespread interest. For example, the transport of electron holes has been studied in DNA because of its relation to oxidative damage, but also because DNA might provide interesting prototypes for nano-electronics. The defect structures induced by UV irradiation of DNA shown in the figure below play a crucial role in explaining the ultrafast pump-probe experiments. The goal of the present investigations is to provide a picture of various defects as complete as possible based on extended ab initio calculations. The main features covered in the present contribution are (i) calculation of the electronic absorption spectra of stacked nucleobases in a DNA environment and analysis in terms of local, excitonic and charge-transfer excitations, (ii) stabilization of the charge-transfer (excimer) states by geometric relaxation and (iii) charge migration including nonadiabatic surface-hopping dynamics in model stacked ethylene systems.