  A relativistic and non-local model for the coherent photoproduction of pions
on spin-zero nuclei is presented. The production operator is derived from an
effective Lagrangian model that contains all the degrees of freedom known to
contribute to the elementary process. Using the framework of DWIA, the matrix
elements of this operator are evaluated using relativistic bound state wave
functions. Final state interactions are accounted for via a pion-nucleus
optical potential. The effects of in-medium modifications of the production
operator are discussed. We give results for $^{12}C$ and $^{40}Ca$, and compare
our calculations to the data of the A2 collaboration at MAMI.
