Abstract: A brief survey of the theory of X－ray diffraction in crystals is given. Different forms of geometrical diffraction theory, i.e., the Laue equation, Bragg equation and Ewald diagram are introduced and their similarities and differences discussed. The kinematical diffraction theory and different forms of dynamical diffraction theory, i.e., Darwin′s simplified treatment, Ewald′s formulation and Laue's formulation, as well as their physical consequences are discussed, emphasis is put on the energy gap, predicted by dynamical diffraction theory, created by the propagation of electromagnetic waves in periodic structures.