This article presents an in-depth analysis of cost-effective energy efficiency and carbon dioxide emissions reduction opportunities in the US iron and steel industry. We show that physical energy intensity for iron and steelmaking (at the aggregate level, standard Industrial Classification 331, 332) dropped 27%, from 35.6 GJ/tonne to 25.9 GJ/tonne between 1958 and 1994, while carbon dioxide intensity (carbon dioxide emissions expressed in tonnes of carbon per tonne of steel) dropped 391c. We provide a baseline for 1994 energy use and carbon dioxide emissions from US blast furnaces and steel mills (SIC 3312) disaggregated by the processes used in steelmaking. Energy-efficient practices and technologies are identified and analyzed for each of these processes. Examination of 47 specific energy efficiency technologies and measures found a total cost-effective reduction potential of 3.8 GJ/t, having a payback period of three years or less. This is equivalent to a potential energy efficiency improvement of 18% of 1994 US iron and steel energy use and is roughly equivalent to 19% reduction of 1994 US iron and steel carbon dioxide emissions. The measures have been ranked in a bottom-up energy conservation supply-curve.