  We study the velocity of bubble walls in the electroweak phase transition.
For several extensions of the Standard Model, we estimate the friction and
calculate the wall velocity, taking into account the hydrodynamics. We find
that deflagrations are generally more likely than detonations. Nevertheless,
for models with extra bosons, which give a strongly first-order phase
transition, the deflagration velocity is in general quite high, $0.1\lesssim
v_w\lesssim 0.6$. Therefore, such phase transitions may produce an important
signal of gravitational waves. On the other hand, models with extra fermions
which are strongly coupled to the Higgs boson may provide a strongly
first-order phase transition and small velocities, $10^{-2}\lesssim v_w\lesssim
10^{-1}$, as required by electroweak baryogenesis.
