The complex welding technology of today demands an understanding of the formulation, manufacture, performance, and use of welding fluxes. The volume of fluxes used in covered-electrode, submerged-arc, flux-cored wire, electroslag, brazing, and oxyacetylene techniques, has grown to a total of probably over 400 million pounds per year in the United States. The fact that in the U. S. today the covering on coated electrodes averages approximately 25% of the 600 to 700-million pound annual production is an indication of the importance of welding fluxes. The technology leading to proper flux formulation has been little understood. It is hoped that a presentation of some of the principles of welding flux technology will provide an appreciation of improved quality of weld metal obtained through slag/metal reactions. The choice of compounds and the exact formulation in the preparation of a flux will depend upon many factors, including both technical and production economies.