Over the past 20 years, surface hardening by induction has been used to increasing degree. One of the major reasons is the ability to create high heat intensity quickly at very well defined locations on the work piece. This leads to low process cycle time and, thus, high productivity while meeting required specifications. This paper introduces the modern concept of induction surface hardening carbon steel and iron parts based on new knowledge accumulated by Inductoheat, Inc. This information will allow the heat treater to better understand the intricacies of induction surface hardening based on modern theoretical achievements and the latest practical applications.