In crystals, atoms are organized in a periodic array, with a pattern repeating itself over and over again. In order to determine how they are organized, we simply have to use X-rays. The diffracted beam will form what we call a reciprocal lattice, which helps us determine the arrangement of atoms. X-rays are necessary to observe things at the atomic level. Modern techniques of crystallography may also be applied to disordered situations such as liquids or biological objects. There again, we can determine the atom arrangement thanks to diffraction measurement.