Jurgen Fuchs Karlstads Universitet Line defects in a topological phase correspond to surface defects in a three-dimensional topological field theory. The classification and properties of such defects, as well as of the point-like defects at which they end, can therefore be studied in the framework of topological field theory. Among the structures emerging in such an analysis are module and bimodule categories over fusion categories. Investigating these structures also provides structural insights, e.g. there turns out to be an obstruction (taking values in the Witt group of modular tensor categories) to the existence of defects, and symmetries of the bulk theory can be related to the transmission of bulk excitations through invertible defects. As an illustration, I will present a few specific results for twist defects in topological multilayer systems.