The 75 year history of the American Geophysical Union has accompanied great advances in our understanding of the physics and chemistry of the transition zone between the Earth's core and mantle. The core-mantle boundary (CMB) is the most significant internal boundary within our planet, buried at remote depths and probably forever hidden from direct observation; yet this region is very important to our understanding of the dynamic Earth system. The thermal and chemical processes operating near the CMB have intimate relationships to fundamental events in Earth history, such as core formation, and continue to play a majore role in the planet's evolution, influencing the magnetic field behavior, chemical cycling in the mantle, irregularities in the rotation and gravitation of the planet, and the mode of thermal convection of the Earth. The D' region, comprising the lowermost 300 km of the mantle, is known to be highly heterogeneous in material properties on large and small scales, presumably due to thermal and chemical variations, while the outermost core is much more uniform. The substantial knowledge that we now have about this remote region is testimony to the remarkable progress made in geophysical remote sensing, prompted by prodigious increases in data, computational power, and experimental methodologies used to investigate deep earth structure and processes.

In the past decade in particular there have been unprecedented multidisciplinary advances in understanding the CMB region, and there are excellent prospects for developing a comprehensive understanding of this region in the next few decades. Fundamental issues yet to be resolved include the causes of layering extensively observed in D', whether or not downwelling slabs accumulate at the base of the mantle, whether plumes arise from the CMB region to feed hotspots at Earth's surface, the extent of core-mantle chemical reactions, the relative importance of topographic versus electromagnetic coupling across the CMB, and the degree to which mantle structure influences the geomagnetic field and its reversals. The overview highlights the progress and future directions in geophysical investigations of the CMB region. ¿ American Geophysical Union 1995