Most of the current views on the evolution of passive margins have roots in ideas that were developed before 1930 in the context of continental drift and geosynclinal theory. These ideas include the concept of an Atlantic type of margin formed by rifting and continental drift; the presence of a thick sedimentary deposit beneath the continental shelves; and subsidence in response to such mechanisms as crustal thinning, igneous underplating, thermal contraction, flexure, and sediment loading. As large amounts of new surface and subsurface data were acquired from modern passive margins after World War II, owing to significant advances in technology for geological and geophysical exploration of the ocean basins, these early ideas were strengthened and modified. With the development of plate-tectonic theory, the origin of passive margins as rifted trailing edges of continents became widely accepted, and significant changes in thinking involved the role of passive margins and their implications for large horizontal displacements in the evolution of geosynclines. Within the past decade, a large number of geophysical models have been developed for passive margins that focus once again on the problem of the mechanisms of vertical movements of the Earth's crust. At the same time, new developments in the acquisition and processing of data from ocean basins, especially deep-reflection data, have resulted in major new concepts about the deep structure of passive margins, including recognition of the importance of underplating and plutonic activity in the thinned rifted crust and the unexpected degree of faulting and formation of horizontal reflectors in the lower continental crust and subcrustal lithosphere.