Induction hardened shafts work well in torsional applications because both the applied stress and the strength are greatest at the surface. In this study test shafts made from SAE 1040, 1541, 1050M, and 4140 steel were induction hardened to different case depths and torsionally tested. Torsional ultimate testing was performed in order to establish a correlation with case depth. Torsional fatigue testing was also performed to establish a relationship with case depth and with torsional strength. Torsional strength increases with increasing case depth but only to a certain point. Increasing the case depth beyond that point results in no further increase in strength. Torsional fatigue life does correlate to case depth and torsional strength to some degree but there is considerable scatter or variability.