In plastic flow of polycrystalline materials above about 30 - 50% of their absolute melting temperature, deformation mechanisms divide into two groups, lattice mechanisms and boundary mechanisms, based on whether the individual processes are independent of, or dependent on, the presence of grain boundaries (Langdon, 1975, 1981; Langdon and Vastava, 1982). With lattice mechanisms, deformation occurs by processes taking place within the grains. These mechanisms are possible in both polycrystalline materials and single crystals. With boundary mechanisms, deformation occurs by processes associated with the presence of grain boundaries. These mechanisms are possible only in polycrystalline materials (Langdon, 1975). Grain boundary sliding and diffusional creep are two major processes of boundary mechanisms. Although it is usually difficult to find clear microstructural evidence for grain boundary sliding and diffusional creep in naturally or experimentally deformed rocks (Schmid et al., 1977; Schmid, 1982; Behrmann, 1985), it has been suggested that grain boundary sliding can be the dominant deformation mechanism in some cases, particularly in fine-grained mylonites or ultramylonites (e.g. Behrmann, 1985; Behrmann and Mainprice, 1987; Stünitz and Fitz Gerald, 1993; Fliervoet et al., 1997).

In this paper, grain boundary sliding and associated accommodation mechanisms will be reviewed with some clear examples from in-situ deformation of an organic analog material. The recognition of grain boundary sliding in naturally deformed rocks will also be discussed. Most of the contents and experimental examples in this paper were already published by Ree (1994). The major addition of this chapter is the time-lapse movies of the experimental examples from Ree (1994).