CARBON AND LOW-ALLOY steels represent over 95% of the construction and fabrication metals used worldwide. Good mechanical properties over a wide range of strengths, combined with relatively low cost and ease of fabrication, account for their widespread usc. Because of these attributes, carbon and low-alloy steels are an excellent choice for appliances, vehicles,bridges, buildings, machinery, pressure vessels, offshore structures, railroad equipment, ships, and a wide range of consumer products. The extensive use of carbon and low. alloy steels means that welding, brazing and ther-mal cutting of such steels is quite common. This chapter provides information on: steel compositions and prop-erties; weldability considerations; good practices and procedures for welding, thermal cutting, and brazing of carbon and low-alloy steels, with guidance on 1 how to avoid problems when welding these steels; and typical applications.