This book reviews the state of the art for determining the "real" structure of matter. Nature does not stack atoms up in crystals in a perfect manner. Various types of "mistakes" find their way into the arrangements of atoms in real crystals. These mistakes or defects determine the physical properties of a material and understanding them is critical to predicting a new materials properties. This book reviews the principal characterisation technique permitting us to measure the defect solid state: X-ray diffraction.