The physical properties of materials depend not only on the interatomic forces between atoms, but also on their microstructure. Much of the progress in Material Science has been based on the formulation of this understanding and the development of processing techniques allowing control of this microstructure. The last decades have seen remarkable developments in technological advances based on new materials and processing techniques. These have revolutionized materials science, leading to novel forms of matter with unprecedented mechanical, chemical, magnetic, optical, and electrical properties.

The relationship between properties and structures, from the sub-nanometre scale of the crystal lattice and dislocations, to coarser precipitates and crystallites, is under continuous study. Understanding the microstructure allows us to change properties in amazing ways. Nowadays, there is also a major effort to develop environmentally friendly and recyclable materials.

Considering the importance of the microstructure, it is clear that Electron Microscopy, which is the dedicated technique for this investigation, is also of great importance. Electron Microscopy, and especially its Transmission mode, offers a lot of possibilities for a direct observation of the structural imperfections caused by chance or purpose. The introduction or the annihilation of lattice defects is nowadays one of the two available methods of properties modification of crystalline materials. The other method, which is the alloying of two or more components, also needs the help of Electron Microscopy, especially its high resolution, as well as its analytical mode.

On the other hand, considering the size of the materials needed for the most recent applications, Electron Microscopy covers the requirements of their investigation in nanometer scale, reaching the level of atomic resolution.

All these lead to the conclusion that Electron Microscopy is not only a necessary complementary technique, but in some cases, the only one able to analyze and determine the origin of a certain property, leading to an important application of a specific material.