Perhaps the least ambiguous signal that the mantle is convecting comes
from observations of seismic anisotropy---the variation of wave speed
with direction---which must arise due to the ordering of material as
deformation occurs. Therefore significant effort has been made over many
years to infer the direction and nature of mantle flow from these data.
Observations have focussed on the boundary layers of the mantle, where
deformation is expected to be strongest and where anisotropy is usually
present. While prospects for mapping flow seem good, the lack of
knowledge of several key issues currently holds progress back. These
include the cause of anisotropy in the lowermost mantle, the causative
material's response to shear, and the single-crystal or -phase seismic
properties of the causative materials. In this chapter we review recent
observations of lowermost mantle anisotropy, constraints on mineral
elasticity and deformation mechanisms, and challenges in linking
geodynamic modelling with seismic observations.