This paper deals with the mathematical theory of invariant curve evolu
tion. We present a high-level procedure for the formulation of geometr
ic heat flows which are invariant with respect to a given Lie group. T
his approach is based on the classical theory of differential invarian
ts. The affine group is then analyzed in detail. Indeed, we give a rat
her complete description of the properties of the affine geometric hea
t equation. We moreover extend the results of [38] from the convex to
the nonconvex case. The paper concludes with a summary of recent appli
cations of curve evolution theory to image analysis.