In this talk I will discuss cosmological first-order phase transitions. These phase transitions which proceed through the nucleation and merger of bubbles on the new phase are known to source gravitational waves. If one of these events occurred in the early universe then upcoming space based gravitational wave detectors like LISA may be able to detect the resulting gravitational wave background that remains today. In this talk I will focus on transitions in which the bubble wall accelerates until collision. In this case, collisions of the bubble walls form the dominant source of gravitational wave production. I will present our results from classical lattice simulations of colliding bubbles of the true vacuum, with a focus on the gravitational wave power spectrum.