What it means for something to be joined in “the solid state” is supposedly well defined. Solid state adhesion of metals is achieved when clean, mated surfaces come together, and can share electrons, effecting metallic bonds. In effect, the metallic bonds shared across the interface are then no different than the ones between specific metal atoms in the parent material matrices. Solid state welding processes are then defined by the mechanisms that we actually manage to achieve such clean surfaces in intermetallic contact. Academically, two such mechanisms are discussed. The first is through forging. Forging facilitates disruption of any surface oxides to expose clean base metal substrate atoms, and by brute force creates intermetallic contact of these atoms. In addition, application of heat affects the thermodynamics of any remaining oxide particles, causing dissociation and additional creation of clean surfaces for inter-atomic electron sharing and bonding. Solid state welding processes are then defined by the physical means by which heating, forging, or both, are applied to the workpieces. Technologies now defined as solid state range from diffusion bonding, to flash butt welding to friction processing to projection welding to high frequency tube welding to magnetic pulse welding. But exactly how solid state are these processes? Diffusion bonding often takes advantages of so-called “diffusion aids”. These diffusion aids actually are melting point suppression additions that promote the formation of liquid films, promoting surface interaction and eventual bonding. Flash butt welding uses the formation of molten films as both a heat generation mechanism and a vehicle for achieving proper alignment before forging. While these liquids are nominally expelled on forging, their artifacts are a key metallurgical component of this type of joint. In solid projection welding, there is a fine line (often crossed) between simply heating and forging, compared to surface melting for attachment. Even in processes such as magnetic pulse welding and explosion bonding, it is now understood that thin retained films of liquid metal (typically less than 5-μm thick) are a key (and sometimes the only) component in attachment of the metal substrates. In short, the line between “solid state” and “fusion” welding is a grey one at best. Most process technologies (with the exception of perhaps cold welding) offer considerable overlap of these bonding mechanisms. This overlap is however, often used to the advantage of those who would apply these technologies, and is often a key factor in their successful utilization.