This issue of the MRS Bulletin aims to highlight the innovative and exciting materials science research now being done using in situ electron microscopy. Techniques which combine real-time image acquisition with high spatial resolution have contributed to our understanding of a remarkably diverse range of physical phenomena. The articles in this issue present recent advances in materials science which have been made using the techniques of transmission electron microscopy (TEM), including holography, scanning electron microscopy (SEM), low-energy electron microscopy (LEEM), and high-voltage electron microscopy (HVEM).The idea of carrying out dynamic experiments involving real-time observation of microscopic phenomena has always had an attraction for materials scientists. Ever since the first static images were obtained in the electron microscope, materials scientists have been interested in observing processes in real time: we feel that we obtain a true understanding of a microscopic phenomenon if we can actually watch it taking place. The idea behind “materials science in the electron microscope” is therefore to use the electron microscope—with its unique ability to image subtle changes in a material at or near the atomic level—as a laboratory in which a remarkable variety of experiments can be carried out. In this issue you will read about dynamic experiments in areas such as phase transformations, thin-film growth, and electromigration, which make use of innovative designs for the specimen, the specimen holder, or the microscope itself. These articles speak for themselves in demonstrating the power of real-time analysis in the quantitative exploration of reaction mechanisms.The first transmission electron microscopes operated at low accelerating voltages, up to about 100 kV. This placed a severe limitation on the thickness of foils that could be examined: Heavy elements, for example, had to be made into foils thinner than 0.1 μm. It was felt that any phenomenon whose “mean free path” was comparable to the foil thickness would be significantly affected by the foil surfaces, and therefore would be unsuitable for study in situ. However, technology quickly generated ever higher accelerating voltages, culminating in the giant 3 MeV electron microscopes. At these voltages, electrons can penetrate materials as thick as 6–9 μm for light elements such as Si and Al, and 1 μm for very heavy ones such as Au and U.
