Evidence of seismic anisotropy in the D″ region holds tantalizing insights into the mineralogy and dynamics of this core mantle boundary layer. In this chapter we summarize the observations to date, characterize the type of anisotropy and finally consider the physical processes that may have produced this seismological signature. Although studies of D″ anisotropy are still a long way from achieving global coverage, it is clear that there are strong lateral variations in the degree of anisotropy, with large regions appearing to be isotropic. Reliable interpretations of this anisotropy require corrections for receiver-side upper-mantle anisotropy and complications due to source side anisotropy can be mitigated using only deep (>500 km) events. To date, regions below the Caribbean and the north Pacific/Alaska show clear evidence for D″ anisotropy, while observations in regions beneath the central Pacific are less straightforward. The primary seismic constraints in the Caribbean and Alaskan regions are: (1) S/ScSH-phases arrive earlier than S/ScSV-phases (2) SKS does not appear to be sensitive to D″ anisotropy (3) these are regions of high seismic velocities and a D″ discontinuity. Transverse isotropy best describes the style of anisotropy in these regions. Two candidate anisotropy mechanisms are considered: (1) that due to lattice-preferred orientation (LPO) of constituent minerals and (2) that due to a shape preferred orientation (SPO). It is difficult to explain the observations with the LPO mechanism. We use effective medium modeling to investigate the more likely SPO anisotropy, which is attributed to oriented inclusions within a matrix of contrasting seismic properties. The seismic constraints force us to conclude that the inclusions must be horizontally aligned tabular bodies (disks or layers). It seems unlikely that the physical process responsible for this anisotropy is associated with infiltration of core material. Instead, there are a number of arguments which suggest that the anisotropy is associated with the hypothesis that, in places, D″ represents a graveyard for subducted material. High aggregate shear velocities can be explained by the retained thermal anomaly of the slab and the anisotropy can be explained by contrasts in the material properties between what was formerly oceanic-crust and oceanic-mantle-lithosphere.