Since transmission electron microscopy (TEM) was first applied to study metals and alloys, an extensive amount of knowledge has been obtained on microstructures that govern the strength, ductility toughness, workability, and other properties of materials. Particularly analytical transmission electron microscopy (ATEM) and its combination with scanning transmission electron microscopy (STEM), energy dispersive spectrometry (EDS), and more recently electron energy loss spectroscopy (EELS) have all made it possible to gain an insight into the crystal structure, morphology, and the chemical composition of fine precipitate phases of the order of 20 nm. In recent years the transmission electron microscopes have reached a stage where observation of individual atoms is possible. In spite of such developments, there are many problems which remain unsolved by electron microscopy. A typical example is the analysis of precipitates in the secondary hardening of steels, which are very difficult to distinguish because of the high dislocation density in the matrix. The structure and composition of these precipitates cannot be identified because they are too small for electron diffraction and for EDS.