The region considered under the general term 'Kopet Dagh' is located east of the Caspian Sea, and includes Northeast Iran and southern Soviet Turkmenia. The regional tectonics are reviewed with special emphasis on the post-Alpine 'Diagonal Fault System'. The seismicity over the last 100 years is studied, and the four strongest earthquakes in Northeast Iran, i.e. 1871/2, 1893 and 1895 Quchan and 1929 Baghan-Germab, are described in detail for the first time on the basis of new bibliographical and field data. These four earthquakes were located on the NNW-SSE 'Bakharden-Quchan Zone', which forms part of the Diagonal Fault System. The 1929 earthquake in particular was accompanied by a surface fracture over 50 km long caused by reactivation of one of the faults of this Zone. Russian work on the seismotectonic aspects of the 1948 Ashkhabad earthquake which occurred in an adjoining zone, and migration of seismic activity in the Kopet Dagh since about 1870, are examined. The overall seismotectonics of the Kopet Dagh are interpreted in terms of an eastern 'NNW trend' which is separated by a longitudinal zone of relative quiescence near 56-57 degrees E from a western 'NNE trend'. Active surface structures throughout the region are on average consistent with a tectonic model based on a NNE motion of Iran with respect to the Turan Plate. Many tectonic features are characteristic of the margins of converging continental plates.