Disclosed is a process and apparatus for induction hardening of the flank, root and tip surfaces of gear teeth so as to provide a surface-hardened layer along said surfaces. An electrically energizable inductor head carries out movement along the space between a pair of adjacent gear teeth, the inductor head having an external profile matching the profile of the space between the gear teeth. Guides maintain clearances between the inductor head and the tooth surfaces, during movement of the inductor head, the clearance being a minimum between the inductor head and the root surface and varying in predetermined manner along the flank surfaces to achieve heating along the flanks and root surfaces to a uniform predetermined temperature. Jets of coolant are directed to the surfaces of the unheated flanks, tips, heated flanks and root of the pair of gear teeth to achieve rapid cooling of said surfaces. The gear is immersed in mineral oil or other non-aqueous quenching agent. The height of the inductor head is less than the depth of the space between the root and tip surfaces of each pair of gear teeth. The operating parameters of (a) the predetermined clearances, (b) the rate of electrical energy supplied to, and the speed of movement of the inductor head and (c) the rate of cooling, are controlled to form a uniform thickness surface-hardened layer from tip to tip along the flanks and root surfaces of the pair of gear teeth, the surface-hardened layer having an internal structure which imparts the equivalent of a built-in compressive stress in its surface, at least in the region of the root curve and its transition to the flank surfaces. This compressive stress enables each tooth to withstand tensile stresses which are applied to gear teeth, when intermeshing gear teeth apply fatigue bending loads to each other.