Topological defects can play an important role in cosmology. In this talk I will discuss the impact of defect formation in the context of first order phase transitions, by which defects can act as local impurities catalyzing the decay of the false vacuum. As concrete particle physics realizations I will consider one of the simplest extensions of the Standard Model, the xSM, where domain wall configurations associated to the new singlet scalar are shown to enhance the tunneling rate (2203.16450), as well as the minimal KSVZ implementation of the QCD axion in the post-inflationary scenario where axion strings can affect the electroweak phase transition in a similar way (2405.08060). I will ...