  It is by now well known that the Poincar\'e group acts on the Moyal plane
with a twisted coproduct. Poincar\'e invariant classical field theories can be
formulated for this twisted coproduct. In this paper we systematically study
such a twisted Poincar\'e action in quantum theories on the Moyal plane. We
develop quantum field theories invariant under the twisted action from the
representations of the Poincar\'e group, ensuring also the invariance of the
S-matrix under the twisted action of the group . A significant new contribution
here is the construction of the Poincar\'e generators using quantum fields.
