  Gravitational waves potentially represent our only direct probe of the
universe when it was less than one second old. In particular, first-order phase
transitions in the early universe can generate a stochastic background of
gravitational waves which may be detectable today. We briefly summarize the
physical sources of gravitational radiation from phase transitions and present
semi-analytic expressions for the resulting gravitational wave spectra from
three distinct realistic sources: bubble collisions, turbulent plasma motions,
and inverse-cascade helical magnetohydrodynamic turbulence. Using
phenomenological parameters to describe phase transition properties, we
determine the region of parameter space for which gravitational waves can be
detected by the proposed Laser Interferometer Space Antenna. The electroweak
phase transition is detectable for a wide range of parameters.
