The Equinox Transition Study (ETS) of September 17--24, 1984, was an extraordinarily successful coordinated campaign. The experimental design resulted in a large quantity of high-quality data that were comprehensive both in their completeness of measured parameters and in coverage that was global rather than mesoscale. Nature also cooperated in providing a calibrated geophysical scale over an extreme dynamic range of disturbance. The 8-day period involved the most disturbed day of the year and a day half as disturbed, separated and preceded by quiet days. The ETS data set was thus unique among campaigns of this nature. The broad scientific aim of ETS is to understand the electromechanical response of the thermosphere/ionosphere system to variable high-latitude forcing. The interdisciplinary cross talk engendered by ETS has led beyond the original goals of the project to the identification of important new couplings between the mesosphere/lower thermosphere/upper thermosphere/ionosphere/magnetosphere through a variety of mechanisms ranging from mechanical to chemical to electrical. In this paper we present an overview of the ETS, and we describe the development of the experimental and analysis philosophies behind the project. Many of the lessons learned are relevant to the planning of future campaigns. Finally, we summarize some of the results obtained from the ETS. ¿ American Geophysical Union 1989