In the Standard Model, the electroweak phase transition is a crossover. In many extensions, including those with additional charged Higgs bosons, the phase transition can be of first order – even strongly so. In such a phase transition, the resulting collisions of bubbles of the new Higgs phase, and the associated interactions of sound waves in the plasma, are substantial sources of gravitational waves. For a phase transition at or around the electroweak scale, these gravitational waves may be detectable by future or planned missions, such as LISA. This can indirectly provide a complementary probe of particle physics beyond the Standard Model. I will review the physics that makes this possible.