From the standpoint of the method of phase diagrams of martensitic transformations (PDMF), an analysis of the literature data, obtained experimentally and theoretically in the study of martensitic transformations in carbon steels, is performed starting from the 20s of the last century and ending with modern works. In accordance with the PDMF, it is impossible to consider polymorphic phase transformations (including martensitic ones) without taking into account the participation of vacancies in them. Just vacancies are the connecting link that unites volume, deformations, and stresses into a single whole. A comparison of the known data on the changes in the crystal lattice parameters of austenite, ferrite, and martensite allow us to conclude that, during the formation of a martensitic crystal, carbon simply shifts in all cells along the edge of the quenching stress that is favourably oriented relative to the current gradient. In this case, the tetragonality of martensite in carbon steels is determined not by carbon, but by vacancy-carbon complexes, which arise as a result of, for example, quenching. A scheme for determining the waves of concentration inhomogeneities in carbon steels is proposed, which made it possible to construct the dependence of the temperatures of martensitic transformations ($M_s$ and $M_f$) on the carbon concentration. Based on these constructions, it was shown that martensite cannot contain carbon more than 1% wt. The PDMF method allows dividing the process of martensite formation into two stages. The first stage is the formation of elastic ‘austenitic’ martensite ($\gamma^{'}$-phase) by shear during cooling. The crystal is deformed until the $\gamma^{'}$-phase remains stable. The second stage is the loss of stability, the isolation of the crystal and its transformation into a ‘ferritic’ ($\alpha^{'}$-phase) martensite. The second stage is a quantum mechanical phenomenon, since the process is caused by a change in interatomic bonds. The process proceeds instantly, with a simply shift and at a constant temperature.