The Great Glen Fault Zone (GGFZ) is a major, reactivated strike-slip fault within the lower Paleozoic Caledonian orogenic belt of the British Isles. The Late Devonian to Tertiary reactivation history is well documented, but the early history is poorly known and controversial. New analysis of early structures and fabrics shows that the GGFZ comprises a 3-km-wide belt of fault rocks ranging from rare mylonite and quartz blastomylonite to common cataclasite, hydrated cataclasite, and phyllonite. Kinematic indicators demonstrate a sinistral sense of displacement with a southeasterly component of downthrow. Microstructural analysis indicates that presently exposed levels of the GGFZ developed initially at depths of about 9--16 km, within the frictional-viscous creep transition zone. Deformation continued during exhumation and was associated with pervasive syntectonic infiltration of hydrous fluids which resulted in widespread retrogression along the GGFZ. Strong protolith mineral phases were progressively replaced by secondary fine-grained aggregates of phyllosilicates leading to a permanent weakening of the fault zone. Relationships between fault zone structures, dated igneous intrusions, and posttectonic (Devonian/Old Red Sandstone) sedimentary rocks constrain sinistral displacement to a late stage in the Caledonian orogeny, between about 428 Ma and 390 Ma. Early displacements probably overlapped crustal-scale, foreland-directed thrusting which resulted from the oblique collision of the Scottish segment of Laurentia with Baltica. There is no geological evidence that these late Caledonian displacements involved reactivation of an older structure at the current exposure level. However, the Caledonian structural architecture of the GGFZ exerted a fundamental influence on the upper crustal Late Devonian to Tertiary reactivation history during which microfracturing and development of discrete fault planes were localized either within or along the boundaries of preexisting belts of cataclasite and phyllonite. ¿ 1999 American Geophysical Union