I obtain an exact solution of Einstein's equations representing the gravitational field of a steady beam of light. Another exact solution representing two parallel beams shining in the same sense is also given; they do not interact. From a study of null geodesics I conclude that a uniform beam of light is gravitationally stable.The exact solutions are plane-fronted gravitational waves. It seems that a large class of these waves have as their sources pulses and beams of light.