The strengthening of low-carbon martensitic steels at stages of uniform and localized deformation has been considered. The experimental data have been approximated using the Ludwik equation and linear dependence. The micro- and fine structure of experimental heats of steels and characteristics of strength, ductility, and toughness have been presented. True strain-true stress curves have been constructed. Strengthening indices for stages of uniform and localized deformation have been calculated. The procedure of calculating the plastic-deformation zone in front of the crack tip has been suggested.