Continental rifling and associated processes of lithospheric extension have been among the most active research frontiers in the Earth sciences over the past four decades. These subjects cover a wide range of interconnected fundamental processes occurring in an important part of the Earth for mankind-- the continental crust and lithosphere--and have captured the interest and attention of researchers from a wide spectrum of disciplines in the solidearth sciences. These disciplines range from classical field geology through newly developed techniques and capabilities for seismological and electromagnetic probing of the Earth, to sophisticated and realistic dynamical modeling with high-speed supercomputers. For more than 30 years, working groups and study committees composed of international researchers focusing on the problems of the Earth's rift zones have been organized under the aegis of programs promoting international cooperative research in the geosciences, such as the International Upper Mantle Project, the Geodynamics Project, and the International Lithosphere Program.