Our ability to observe atomic-scale features of grain boundaries has tremendously improved during the past decade. In this paper we give, aided by a number of examples, a select overview, on progress in the field of grain boundary research directly related to the advent of modern high-resolution electron microscopy (HREM) instruments (point-to-point resolution better than 0.2 nm). Examples of grain boundary issues addressed by atomic structure observations of grain boundaries in oxides and metals will be given with emphasis on systematic investigations of the role of macroscopic and microscopic grain boundary parameters. Since comparisons between observed interface structures and atomistic computer modeling results are quite important, considerable efforts towards quantification have been undertaken recently by a number of authors. Most valuable insights have been obtained by the systematic examination of a range of grain boundary structures, using a combination of experimental observations and computer modeling results. In this manner HREM observations have been invaluable not only as a test of theoretical models, but also by exposing common atomic-scale features of high-angle grain boundaries. This has brought us closer to the goal of generating a general understanding of the interface structure and its connection to properties. Such studies have given valuable insights regarding the correlations between macroscopic grain boundary geometry, interfacial energy, and atomic relaxation modes.
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