  A covariant, unitary and gauge invariant theory for pion photoproduction on a
single nucleon is presented. To achieve gauge invariance at the operator level
one needs to include both the $\pi N$ and $\gamma\pi N$ thresholds. The final
amplitude can be written in terms of a distorted wave in the final $\pi N$
channel provided one includes additional diagrams to the standard Born term in
which the photon is coupled to the final state pion and nucleon. These
additional diagrams are required in order to satisfy gauge invariance.
