Various aspects of gravitational wave propagation in plasmas and vacuum are discussed. First, we analyse single particle trajectories, study the resonant interaction of gravitational waves with photons, and show that photons can be strongly energized by gravitational waves. Second, this exchange of energy between the photons and the gravitational waves is treated statistically, using a kinetic equation for photons. Gravitational wave instabilities induced by intense photon beams are considered. The effects are very much dependent on the gravitational wave dispersion in the plasma medium, and in particular, on its turbulent state.