The interaction of a high power laser beam with matter results in a highly ionized medium, called a plasma. For target irradiations higher than 10 13 Wcm −2 , the energy density supplied to the target is such that the plasma temperature exceeds one million degrees. The major part of energy is then thermal but also radiative and the maximum of the emission (1A < λ max < 100A) appears in the X-ray range. It is important to study this radiation, because it is the image of the physical processes which control the interaction between electromagnetic wave and matter and it contributes for an important part to the energetic balance. Furthermore, this X-ray emission can be used as a source in the studies of inertial confinement by indirect drive, in atomic physics, in absorption spectroscopy, in X-ray shadowgraphy, in X-ray laser, and industrial applications like lithography. The studies of the conversion efficiency of laser energy in X-ray energy have shown the advantage of short laser wawelengths, of high Z materials, and the appearance of a maximum of this conversion as a function of laser irradiance. For gold targets, 0.35μm laser wavelength and 10 14 Wcm −2 laser flux, the X-ray conversion efficiency can reach 60%