  This paper defines a parabolic frequency for solutions of the heat equation
along homothetically shrinking mean curvature flows and proves its monotonicity
along such flows. As a corollary, frequency monotonicity provides a proof of
backwards uniqueness. Additionally, for solutions of more general parabolic
equations on mean curvature flow shrinkers, this paper provides bounds on the
derivative of the frequency, which similarly imply backwards uniqueness.
