INTRODUCTIONThe process of brazing that we know today began as an ancient art. What began as art@ however@ evolved through our increased understanding of the nature and behavior of materials into art@ technology@ and science. In a very general sense@ brazing is a process for joining materials which relies on the melting@ flow@ and solidification of a filler metal to form a leaktight seal@ a strong structural bond@ or both. The uniqueness of the process is that metallurgical bonds are formed during brazing by melting only the filler metal and not the parts being joined. Brazing is a well established commercial process@ and is widely used in industry@ in large part@ because almost every metallic and ceramic material can be joined by brazing. Generally@ brazing can easily be performed by manual techniques@ but@ in many cases@ it can just as easily be automated if necessary.The American Welding Society defines brazing as ??a group of welding processes which produces coalescence of materials by heating them to a suitable temperature and by using a filler metal having a liquidus temperature above 840??(450?? and below the solidus temperature of the base materials. The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction.?? This definition serves to distinguish brazing from other joining processes of soldering and welding. Brazing and soldering share many important features@ but the term ??brazing?? is used when the joining process is performed above 840??(450??@ while ??soldering?? is used below that temperature. Brazing differs from welding in that in braze processing the intention is to melt only the braze filler metal and not the base materials. In welding@ both filler metals and base metals are melted during the process.To achieve a good joint by any variation of the brazing process@ the parts must be properly cleaned and must be protected from excessive oxidation by fluxing@ or by use of a controlled atmosphere. In addition@ the parts must be designed so that when they are properly aligned a capillary is formed in which the molten filler metal can flow. Also@ a heating process must be selected that will produce the proper brazing temperature and heat distribution. The various brazing processes@ joint design@ cleaning and heating methods@ and details specific to particular materials are outlined in this Brazing Handbook. The purpose of this chapter is to provide a basic understanding of the brazing process through a review of the factors fundamental to the process itself. To assist the reader with unfamiliar terms@ a glossary of terms commonly used in the field is provided in Appendix A.