  A model for the gamma p ----> pion+ pion- p reaction developed earlier is
extended to account for all isospin channels. The model includes N,
Delta(1232), N^*(1440) and N^*(1520) as intermediate baryonic states and the
rho-meson as an intermediate two pion resonance. Although many terms contribute
to the cross section, some channels exhibit particular sensitivity to certain
mechanisms of resonance excitation or decay and the reactions provide novel
information on such mechanisms. In particular the gamma N ----> N^*(1520) ---->
Delta(1232) pion process affects all the channels and is a key ingredient in
the interpretation of the data. Comparison is made with all available data and
the agreement is good in some channels. The remaining discrepancies in some
other channels are discussed.
