X-rays of a suitable wavelength range for protein crystallography (~0.8 - 2.3 Å) are generated by three commonly used devices: X-ray tubes, rotating anodes and synchrotrons. In-house or laboratory sources will produce X-rays using either an evacuated tube or a rotating anode. X-ray tubes consist of a filament that acts as a cathode. Electrons are emitted by the glowing cathode and accelerated by several tens of kVs across the vacuum towards the anode, which consists of a metal target made of a characteristic material, usually copper or chromium, for protein crystallography. As the electron beam impacts the anode, the high kinetic energy of the electrons is converted during deceleration into X-rays producing a) a continuous spectrum consisting of bremsstrahlung ("braking radiation") and b) emission lines characteristic for electronic transitions caused in the anode material. The characteristic X-ray emissions, which are important for crystallography, have an intensity that is several orders of magnitude higher than the bremsstrahlung. The Ka1 and Ka2 components of the X-rays emission are cut out from the bremsstrahlung and other emission lines by filters, monochromators or X-ray mirrors, and the resulting monochromatic X-rays are collimated and focused onto the crystals. When X-rays are produced by a rotating anode, the cathode and anode are housed under vacuum, in which the anode target rotates at high speed to efficiently distribute and dissipate heat. The wavelength of an in-house source such as a tube or rotating anode generator is fixed by the choice of anode target material and not tunable, as is the case at a synchrotron, and the intensity of the source is less than that of a synchrotron.