Heat treated bar and forging steels remain the major material for parts (such as shafts, gears, bearing, fasteners, oil country tubulars, aircraft landing gear, tools of all sorts, and many other applications) manufactured for demanding applications that require high or ultra-high strength, high fatigue resistance, and /or high wear resistance, all combined with good levels of toughness and fracture resistance. This course will review the phases that form in carbon and alloy steels and the modifications produced in the iron-carbon diagram by alloying and manufacture. Microstructures and the phase transformations by which they are produced in all phases of manufacture, from casting through quench and tempering, will be described and related to mechanical properties and performance, including fracture and potential embrittlement phenomena.