Processing has always been a key component in the development of new materials. Basic scientific understanding of the reactions and transformations that occur has obvious importance in guiding progress. Invaluable insight can be provided by observing the changes during processing, especially at high magnification by in situ microscopy. Now that this can be achieved at the atomic level by using high-resolution electron microscopy (HREM), atomic behavior can be seen directly. Accordingly, many deductions concerning reactions in materials at the atomic scale are possible.The purpose of this article is to illustrate the level reached by in situ HREM. The essential procedure is to form a high-resolution image of a standard transmission electron microscope (TEM) sample and then to alter the structure by some means in a controlled manner, such as by heating. Continual recording on videotape allows subsequent detailed analysis of the behavior, even on a frame-by-frame (1/30 second) basis. The most obvious advantage is to follow the atomic rearrangements directly in real time. However, in addition, by continuous recording no stages in a reaction are missed, which can often occur in a series of conventional ex situ annealed samples because of the limited number of samples that can realistically be examined by HREM. One can be sure that the same reaction, in the same area, is being studied. Furthermore, by changing the temperature systematically, extremely precise kinetic measurements can be made (e.g., for activation energies and kinetic laws) and the whole extent of a material transformation can be investigated in one sample, something that would take months of work if studied conventionally. The information provided by in situ HREM is often unique and so it can become an important technique for fundamental materials investigations.