The structure of grain boundaries and the influence of solute segregation on boundary structure were studied using diffraction and microscopy techniques. The determination of the projected atomic structure of a large angle [001] twist boundary by use of X-ray diffraction techniques was demonstrated. Using an electron diffraction technique it was shown that the local change in plane spacing at a large angle [001] twist boundary was dependent on the type of bond (ionic, metallic, covalent) in the material. It was shown that solute segregation causes a major change in the dislocation content of a small angle [001] twist boundary in Fe containing a small amount of Au. These observations are evidence for the occurrence of a two-dimensional phase transformation in the grain boundary. As a result of these studies, generalizations were made concerning the structure of large-angle twist boundaries and the influence of bond type on boundary structure.