This monograph provides an overview of the progress made in illuminating the properties of deep slabs and the surrounding mantle, since the introduction of the plate tectonics model to the earth sciences 25 years ago. The thermal and chemical characteristics of the subducted lithosphere are determined through thermal and petrological modelling, with seismological observations providing critical constraints on model parameters. Down-wellings of the oceanic lithosphere play a critical role in plate tectonics by recycling to the mantle material that has risen at mid-ocean ridges and cooled at the Earth's surface. To assist future efforts in developing detailed thermal and petrological models of oceanic lithosphere down-wellings, this volume includes a review of seismological observations and models. A range of seismological procedures are considered, from travel time constraints on seismic velocity anomalies in the subducting lithospheric slabs, to wave conversions and reflections of internal and external slab boundaries. A reference list is included for earth science researchers and seismological specialists which lists most of the critical literature on slab structure.