  For stationary harmonic maps between Riemannian manifolds, we provide a
necessary and sufficient condition for the uniform interior and boundary
gradient estimates in terms of the total energy of maps. We also show that if
analytic target manifolds do not carry any harmonic S^2, then the singular sets
of stationary maps are m \leq n - 4 rectifiable. Both of these results follow
from a general analysis on the defect measures and energy concentration sets
associated with a weakly converging sequence of stationary harmonic maps.
