In X-ray crystallography the structure factor F(hkl) of any X-ray reflection (diffracted beam) hkl is the quantity that expresses both the amplitude and the phase of that reflection. It plays a central role in the solution and refinement of crystal structures because it represents the quantity related to the intensity of the reflection which depends on the structure giving rise to that reflection and is independent of the method and conditions of observation of the reflection. The set of structure factors for all the reflections hkl are the primary quantities necessary for the derivation of the three-dimensional distribution of electron density, which is the image of the crystal structure, calculated by Fourier methods. This image is the crystallographic analogue of the image formed in a microscope by recombination of the rays scattered by the object. In a microscope this recombination is done physically by the lenses of the microscope but in crystallography the corresponding recombination of diffracted beams must be done by mathematical calculation.