  We study singularity structure of Yang-Mills flow in dimensions $n \geq 4$.
First we obtain a description of the singular set in terms of concentration for
a localized entropy quantity, which leads to an estimate of its Hausdorff
dimension. We develop a theory of tangent measures for the flow, which leads to
a stratification of the singular set. By a refined blowup analysis we obtain
Yang-Mills connections or solitons as blowup limits at any point in the
singular set.
