Abstract: A regenerative cavity X-ray lasing oscillator and method in which an X-ray laser target is positioned between a pair of opposed coaxial mirrors at predetermined fixed spacing and forming an X-ray cavity. A series of light pulses is directed onto the target surface at predetermined fixed time intervals that correspond to spacing between the mirrors. Each light pulse ablates a portion of the target surface to form an ion plasma adjacent to the target surface of a character and composition for spontaneous emission of X-ray energy along the axis of the cavity. Such X-ray energy oscillates in the cavity and is amplified upon each passage through the plasma. One of the mirrors is constructed for extracting the amplified X-ray laser energy from the cavity.