AbstractThe determination of the structures of catalysts is an important step in understanding their behaviour and in developing new or improved catalysts. By their nature, catalysts mostly have a structure which can be resolved only by transmission electron microscopy (TEM) used near its limit of applicability. This article discusses recent selected examples of the use of TEM to examine materials used as catalysts, such as clusters of atoms, small crystalline particles, the materials (oxides) on which these are supported, zeolites, and deposits of carbon which often form on catalysts during catalytic reactions. Interpretation of the images is often aided by the techniques of image processing.