  I discuss the generation of a stochastic background of gravitational waves
during a first order phase transition. I present simple general arguments which
explain the main features of the gravitational wave spectrum like the $k^3$
power law growth on large scales and a estimate for the peak amplitude. In the
second part I concentrate on the electroweak phase transition and argue that
the nucleosynthesis bound on its gravitational wave background seriously limits
seed magnetic fields which may have been generated during this transition.
