Coherent motion of particles in a plasma can imprint itself on radiation. The recent advent of high-power lasers—allowing the nonlinear inverse Compton-scattering regime to be reached—has opened the possibility of looking at collective effects in laser–plasma interactions. Under certain conditions, the collective interaction of many electrons with a laser pulse can generate coherent radiation in the hard x-ray regime. This perspective paper explains the limitations under which such a regime might be attained.