We review the current state of the seismological constraints on core structure. The evidence supports a simple two-layer model of the core. The fluid outer core seems to have properties which are consistent with those of a vigorously convecting region. The inner core has a sharp boundary characterised by a 0.5--0.6 g/cc density jump, a 0.6--0.7 km/s jump in P velocity, and a jump in shear velocity to 2.5 km/s or greater. There is weak evidence for an anomalously steep shear velocity gradient at the top of the inner core. Finally, the attenuation of both body waves and free oscillations indicate that the inner core is strongly attenuating near its surface though the data suggests that the attenuation is frequency dependent. ¿ American Geophysical Union 1990