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Diffusion welding is a process in which a weld is made by the simultaneous application of heat and pressure which results in the coalescence of two surfaces by diffusion-controlled processes. Diffusion welding has often been called diffusion bonding, solid-state bonding, solidphase welding, isostatic welding and many other terms. The level of activity in the development of diffusion welding has been high. This interest stems largely from aerospace, electronics and nuclear applications wherein exotic and often difficult to weld materials must be joined. Many high-performance alloys are difficult to weld because of metallurgical reasons. Diffusion welding, which produces no melting, little distortion and much lower temperature exposures than are found in fusion welding, offers many potential advantages over other conventional welding processes. It does have its limitations, many of which relate to economics and geometrical factors involved in complex joints. This report is a review of the present state of the art of diffusion welding. The content is based on an extensive literature search by the authors. It contains a discussion of the mechanisms involved in the process and the metallurgical factors which are involved. It further presents numerous examples of the applications which have been made and some of the results obtained by using diffusion welding.