The phase transformations can take place not only in the bulk of solids but also in grain boundaries (intercrystalline boundaries). Firstly, these are the faceting — roughening phase transitions. They can be of either the first or the second order (continuous). The grain boundary (GB) facets are parallel to the densely packed planes of the coincidence sites lattices (CSL). CSL is the superlattice containing the matching crystal nodes of both grains. As the temperature decreases, the new facets with increasing CSL indices become equilibrium. When the temperature rises, the GB may becoem rough and completely lose its facets. Secondly, these are phase transitions from incomplete wetting of the GB with a melt or a second solid phase to complete one and vice versa. GB wetting phase transitions can also be of first or second order (continuous). In this case, the wetting phase is a "true" bulk phase of arbitrarily high thickness. And finally, thirdly, thin layers of the intergarnular phase can form in GB when the lattices of two neighboring grains still interact with each other through it. This phase (for example, quasi-liquid one) is stable in GB, but unstable in the bulk. It can as a result of pre-wetting, pre-melting, or pseudo-incomplete wetting phase transformations. GB phase transformations affect the properties of the polycrystal as a whole and lead to the appearance of new GB lines in bulk phase diagrams.