Since long, phosphorus has been treated as deleterious to steel, especially, on mechanical properties. For this reason, most of the steel makers control phosphorus content within 0.04 wt.% in steel. However, the main focus of this book is that phosphorus is not harmful on the mechanical properties of steel when used judiciously even if its content is more than 0.3 wt.%. By proper heat-treatment at alpha + gama dual phase region of Fe-P phase diagram and proper design of chemical composition, e.g., by keeping an effective critical carbon concentration of about 0.03 wt.% for displacing phosphorus from the grain boundary of steel, good strength and ductility will be obtained. Another aspect of this book is the corrosion performance of steels added with phosphorus. It is found that phosphorus in the form of phosphate reduced corrosion of steel in concrete environment containing aggressive chloride ions. The corrosion performance of Fe-P alloys in concrete environments have been evaluated by using various electrochemical techniques. However, for realizing the beneficial effect of phosphorus in atmospheric condition, longer time of about 5 to 10 years is required.