The addition of microalloying elements (MAE) to low C-Mn-Si HSLA steels has led to many benefits to the producers,fabricators and end-users.Microstructural improvements such as microstructural refinement,higher dislocation and sub-grain boundary densities and finer M-A-C distributions have led to higher strength,improved toughness and better formability.These improvements can often be traced to the MA addition.In steels for load-bearing applications,the combination of MAE with hardenability additions (Cr,Mo,B,etc.) and lower transformation temperatures has led to much higher strength levels than what were available a few years ago.The resulting nonpolygonal,bainitic and martensitic ferrite microstructures have not only higher strengths but also adequate levels of improved ductility and toughness.Hot strip,plate and pipe applications have benefitted from these developments.Similar improvements have been found in the microalloyed forging steels,where the change from pearlite-ferrite to bainitic ferrite microstructures has led to higher strengths and improved high-cycle fatigue resistance,with little penalty in ductility and toughness.In the cold rolled gauges,both the so-called Advanced High Strength Steels (DP,TRIP and Complex Phase Steels) and the martensitic direct-quenched and press-quenched steels,along with the Interstitial-Free steels,have benefited from MAE additions,especially in the very popular zinc-coated sheet form.This paper will briefly review each of these topic areas,and the underlying physical metallurgy will be discussed. The addition of microalloying elements (MAE) to low C-Mn-Si HSLA steels has led to many benefits to the producers, fabricators and end-users. Microstructural improvements such as microstructural refinement, higher dislocation and sub-grain boundary densities and finer MAC distributions have led to higher strength, improved toughness and better formability. the improvements can often be traced to the MA addition.In steels for load-bearing applications, the combination of MAE with hardenability additions (Cr, Mo, B, etc.) and lower transformation temperatures has led to much higher strength levels than what were available a few years ago. caused by nonpolygonal, bainitic and martensitic ferrite microstructures have not only higher strengths but also adequate levels of improved ductility and toughness. Hot strip, plate and pipe applications have benefitted from these developments.Similar improvements have been found in the microalloyed forging steels, where the change from pearlite-ferrite to b ainitic ferrite microstructures has led to higher strengths and improved high-cycle fatigue resistance, with little penalty in ductility and toughness. the cold rolled gauges, both so-called Advanced High Strength Steels (DP, TRIP and Complex Phase Steels) and the martensitic direct-quenched and press-quenched steels, along with the Interstitial-Free steels, have benefited from MAE additions, especially in the very popular zinc-coated sheet form. This paper will soon review each of these topic areas, and the underlying physical metallurgy will be discussed.