This paper concerns a new kind of steel, its development, its metallurgical characteristics, its welding and gas-cutting characteristics, its low-temperature toughness, its applications, and its engineering potentialities. This steel was developed by the United States Steel Company to offer to industry a plate steel having a unique combination of properties, such as a far higher strength than has hitherto been available, superior toughness, and good weldability. As will be evident from the test results which will be described in detail, these rather formidable goals have been achieved. This new USS “Carilloy” steel has been designated “T-1.” It is characterized by yield-strength levels of 100,000 psi and above. At these high strengths it retains its toughness to much lower temperatures than ordinary structural steels. In addition, welds which develop the full strength of the base metal can be made in T-1 steel without preheating or postheating.