Topological defects can act as local impurities that seed cosmological phase transitions. In this talk we will focus on how domain walls can affect the electroweak phase transition in the minimal extension of the SM including a scalar singlet odd under a Z_2 symmetry. When the transition is two-step, the early breaking of the discrete symmetry implies the formation of domain walls, which can then act as seeds for the final step. The rate of the seeded phase transition can be evaluated within a 3d theory on the domain wall surface, and it is generically faster when compared to the homogeneous tunneling. We will comment on phenomenological implications for gravitational waves and baryogenesis as well as possible future directions.