Many solids crystallize into regular structures. In this chapter mechanisms that lead to the binding of atoms to form ordered structures are discussed. The various crystal structures are presented and the notion of the reciprocal lattice is introduced. Methods to study different lattice structures are discussed with focus on diffraction of different radiations, such as X-ray diffraction. Bragg’s law gives the conditions for diffraction and gives information on the geometry of the lattice. Also, information about the basic unit that constitutes a unit cell is considered.