A substantial history of research has established the Ballantrae Complex as an ophiolite assemblage, generated within the Iapetus Ocean and accreted at the margin of Laurentia as that ocean began to close. Radiometric dates and graptolite biostratigraphy provide an age range from Neoproterozoic (rarely and questionably) to Early Ordovician (commonly and securely). Components of the complex formed in a range of oceanic environments: within-plate oceanic islands, island arcs and back-arc spreading zones. Overall, the various lines of evidence point to an early Arenig marginal basin, incorporating some older elements, which was tectonically disrupted and compressed during an obduction event driven by arc–continent collision. Despite the general consensus as to the age and origin of the complex there remain areas of uncertainty. It is highly probable that some components of the complex that are now structurally juxtaposed originated at a considerable distance from each other both horizontally and vertically. Further, the sporadic presence of high-grade metamorphic rocks – notably blueschists – suggests a complicated and probably polyphase structural history, whilst the relative importance of serpentinite diapirism versus compressional and strike-slip tectonics remains unresolved. In terms of regional structure, the complex appears to occupy a steeply inclined fault zone, separating the Midland Valley from the Southern Uplands terrane to the south, to a depth of several kilometres. This situation questions the concept of the complex as the vestige of an ophiolite sheet obducted on to the Midland Valley basement.