  We study the perturbative unitarity of noncommutative scalar field theories.
Field theories with space-time noncommutativity do not have a unitary S-matrix.
Field theories with only space noncommutativity are perturbatively unitary.
This can be understood from string theory, since space noncommutative field
theories describe a low energy limit of string theory in a background magnetic
field. On the other hand, there is no regime in which space-time noncommutative
field theory is an appropriate description of string theory. Whenever
space-time noncommutative field theory becomes relevant massive open string
states cannot be neglected.
