Publisher Summary Welding in the context of this chapter is the joining of two or more pieces of metal so that the parts to be joined merge with one another forming a homogeneous whole across the connection. Welding a metal requires the introduction of energy that can be as heat directly or in a form that will convert to heat where it is required. The chapter describes the principal features of the welding processes applied to the materials that are most commonly used in structural, mechanical, and process plant engineering—namely, steels and aluminum alloys. It reviews manual metal arc welding, submerged arc welding, and gas shielded welding. In manual metal arc welding, the welder holds in a clamp or holder, a length of steel wire, coated with a flux consisting of minerals, called a welding electrode or rod; the holder is connected to one pole of an electricity supply. The metal part to be welded is connected to the other pole of the supply and as the welder brings the tip of the rod close to it, an arc starts between them. Submerged arc welding uses a continuous bare wire electrode and a separate flux added over the joint separately in the form of granules or powder. The arc is completely enclosed by the flux so that a high current can be used without the risk of air entrainment or severe spatter but otherwise the flux performs the same functions as the flux in manual metal arc welding.