The early part of the article traces the implications on planetary cores of various revisions of the writer's original Earth modelsA andB. Recent evidence indicates that it is appropriate to consider at most two-layer cores in the terrestrial planets. Current evidence on equations of state for cores is presented. The later part of the article puts forward a proposal which, though quite distinct from the mantle-core phase transition proposal of some twenty years ago, retains the assumption that the pressurepc at the Earth's mantle-core boundaryN is a critical pressure crucially involved in the changes atN. The proposal envisages an outer core of Fe2O which is unstable at pressures less thanpc. In the form set down, the proposal restores the agreement of the Earth, Mars and Venus with a common overall composition and meets various revisions of earlier planetary and other observational data. Although the calculations so far made are only preliminary, the agreement appears to be very striking. While still requiring the present Mercury to be significantly different from the primitive Mercury, the Fe2O has an advantage over the phase transition proposal in providing for a larger iron core. Brief comments are made on the Moon.