Analytical methods using X-rays are some of the most powerful techniques for materials characterization. The purpose of this chapter is to review various techniques that utilize X-rays for probing molecular and supramolecular structure, in addition to discussing the instrumental aspects that are characteristic for each method. When matter is bombarded with high-energy electrons, protons, deuterons, a-particles, or heavier ions, X-rays are emitted. X-rays usually have a wavelength between 0.05 and 100 Å. They propagate at the velocity of light and travel in straight lines, undeflected by electric or magnetic fields. When X-rays encounter matter, a variety of processes may take place and each of these processes can be utilized to study particular properties of materials. Some of these processes include reflection, refraction, diffraction, scatter, absorption, fluorescence, and polarization.