The development of a commercially successful process for the catalytic synthesis of ammonia was a scientific as well as a technical triumph. Its implications were con­ siderable. It demonstrated the power of a combination of innovative technology and engineering together with basic chemical science, and it introduced ideas and techniques into catalytic science and process engineering which are still with us today. In a real sense, this process changed the face of industrial chemistry and process technology. Of course, the key step in the direct synthesis of ammonia was the development of an efficient catalyst, and the historical account given by Dr. S. A. Topham in the first chapter of this volume shows how this was success­ fully accomplished, and how this was combined with the successful solution of other daunting technical problems to make the overall process possible. The microstructure of a catalyst is an important feature which determines its behaviour, and the electron microscope is one of the most important instrumental methods by means of which structural and microstruc­ tural information can be obtained. Nevertheless, the elec­ tron-optical processes of image formation are complex, but need to be properly understood if image interpreta­ tion is to be done reliably. In the second chapter of this volume, Dr. J. V. Sanders addresses the entire field of the application of electron microscopic methods to the examination of catalysts.