  We provide a comprehensive, up-to-date analysis of possible New Physics
contributions to the mass difference $\Delta M_D$ in $D^0$-${\bar D}^0$ mixing.
We consider the most general low energy effective Hamiltonian and include
leading order QCD running of effective operators. We then explore an extensive
list of possible New Physics models that can generate these operators, which we
organize as including Extra Fermions, Extra Gauge Bosons, Extra Scalars, Extra
Space Dimensions and Extra Symmetries. For each model we place restrictions on
the allowed parameter space using the recent evidence for observation of $D$
meson mixing. In many scenarios, we find strong constraints that surpass those
from other search techniques and provide an important test of flavor changing
neutral currents in the up-quark sector. We also review the recent BaBar and
Belle findings, and describe the current status of the Standard Model
predictions of $D^0$-${\bar D}^0$ mixing.
