  A monochromatic gamma ray line results when dark matter particles in the
galactic halo annihilate to produce a two body final state which includes a
photon. Such a signal is very distinctive from astrophysical backgrounds, and
thus represents an incisive probe of theories of dark matter. We compare the
recent null results of searches for gamma ray lines in the galactic center and
other regions of the sky with the predictions of effective theories describing
the interactions of dark matter particles with the Standard Model. We find that
the null results of these searches provide constraints on the nature of dark
matter interactions with ordinary matter which are complementary to constraints
from other observables, and stronger than collider constraints in some cases.
