The strength of interaction between tectonics, ocean circulation and climate is a major concern of paleoclimate research. To evaluate the strength, we must assess the time of onset and development of the Antarctic Circumpolar Current (ACC) and its likely effects on climate, particularly Antarctic glaciation. Developments in numerical climate modeling, marine geology, tectonics and physical oceanography have cast doubt on widely held assumptions of a causal relationship between the ACC and glacial onset, in the Eocene-Oligocene boundary interval. Here we argue that our best chance to determine ACC onset and development is in the Scotia Sea region ("Drake Passage"), south of South America. There lies the greatest tectonic uncertainty, concerning when a complete deep-water circumpolar pathway was created, and (thus) when the ACC developed as we know it today. There also, the ACC is topographically constrained, and key factors (water mass and sediment distributions, sea-floor spreading history) are sufficiently well known. (modif. j. abstr.)