A novel X-ray diffraction (XRD) theory is intensively revealed in this book. The theory will extend present XRD view from one dimension to two and three dimensions, enabling readers to see the invisible characteristics inside materials. The two-dimension (2D) theory involves two angular variables of the Bragg angle Θ and the angle α, introduced to show the location of sample with respect to incident X-ray beam. Such variables are used to compose a set of general mathematical models, which include a general diffraction intensity equation, an azimuth-angle equation and a common scan mode, as well as the Bragg equation, for both of the surface-and transmission-reflection treatments. This book stresses upon the X-ray analyses for natural and synthetic materials. This book is dedicated to create a bridge between basic texts and specialist works and should be helpful to scholars studying XRD theory.