This unit consists of three issues devoted to describing the structure of materials, starting with the simplest and studying atomic structure and the link to reach the ideal crystals in the first instance and ultimately real, studying defects crystalline. Students may have acquired some of the concepts of item 4 (about atomic structure and link) in previous courses. In this course Fundamentals of materials science is very interesting the role of atomic bonding when a classification of materials. Simplifying we see a rough correspondence between metallic bond, ionic and covalent metal, ceramics and polymers, respectively. In introducing agenda item 5 the crystal structures of many important materials, making a more detailed study of the structures of metal materials, and a descriptive way the different structures of ceramics and polymers. This topic presents final part X-ray diffraction as a useful tool for studying crystal structures. The last issue of this block (item 6) is dedicated to identifying the various imperfections that can be found in the structures studied in the previous topic, and solid state diffusion.