Photo- and electroproduction of mesons in a gauge-invariant framework

A comprehensive formalism is presented to describe the photo- or electropro duction of mesons off the nucleon. Taking the hadrons as extended objects d escribed by form factors, Ne discuss the issues of gauge invariance and of off-shell form factors (both hadronic and electromagnetic). Specifically, w e show how the gauge invariance of the production current is preserved by i ntroducing auxiliary currents which take the place of the exchange currents usually neglected entirely in practical applications. The relation of the present formulation to tree-level-type prescriptions is discussed. Applicat ions to kaon photoproduction are shown.