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A remarkable diversity of geophysical techniques is being used to probe the transition zone between the outer core and the lower mande. This inaccessible region, 2900 km below the Earth's surface, is now recognized to profoundly influence the style of mantle convection, thermal and chemical plume formation, secular variation, and possibly reversals of the magnetic field, core-mantle exchanges of angular momentum, long-wavelength gravitational variations, and chemical evolution of the Earth. Establishing the thermal and chemical structure of the lowermost mande and outermost core is critical to our understanding of the dynamic processes near this transition zone, and progress is now accelerating as a result of an interdisciplinary approach. Recent contributions from seismology, mineral physics, geomagnetism, and geodynamics are synthesized in this article, and a model with a combined thermal and chemical boundary layer at the core-mantle boundary is proposed. The recent formation of the IUGG and AGU committees for SEDI (Studies of the Earth's Deep Interior) is facilitating communication in this and related interdisciplinary efforts to understand how the interior of the planet works.