  We study the perturbative dynamics of noncommutative field theories on R^d,
and find an intriguing mixing of the UV and the IR. High energies of virtual
particles in loops produce non-analyticity at low momentum. Consequently, the
low energy effective action is singular at zero momentum even when the original
noncommutative field theory is massive. Some of the nonplanar diagrams of these
theories are divergent, but we interpret these divergences as IR divergences
and deal with them accordingly. We explain how this UV/IR mixing arises from
the underlying noncommutativity. This phenomenon is reminiscent of the channel
duality of the double twist diagram in open string theory.
