In the last 15–20 years significant success has been achieved in studying the structure and behavior of grain boundaries, and this has led to the appearance of concepts such as grain-boundary dislocations, grainboundary crystal geometry models, and grain-boundary splitting, sliding and migration. The inclusion/matrix boundaries have been studied very little, although the problem of these boundaries is even more complex than the grain boundary problem. The configuration of the inclusion/matrix interphase boundaries in steel and the possible changes of their initial structure in the process of mechanical and thermal treatment are amenable to description in boundary structure defect terms.