  In this paper, we formulate and prove a general compactness theorem for
harmonic maps using Deligne-Mumford moduli space and families of curves. The
main theorem shows that given a sequence of harmonic maps over a sequence of
complex curves, there is a family of curves and a subsequence such that both
the domains and the maps converge off the set of "non-regular" nodes. This
provides a sufficient condition for a neck having zero energy and zero length.
As a corollary, the following known fact can be proved: If all domains are
diffeomorphic to $S^2$, both energy identity and zero distance bubbling hold.
