In this lecture we examine the geometric conditions that lead to diffraction of Xrays by crystals. First, we derive Bragg's Law, which gives the angle at which the scattered Xrays constructively interfere if the Xrays are partially reflected from parallel planes in a crystal. Next, we consider scattering in the forward direction (transmission) of an incoming beam of Xrays by a onedimensional crystal, which gives Laue's equations.