  We develop a relativistic quark model for pion structure, which incorporates
the non-trivial structure of the vacuum of Quantum Chromodynamics as modelled
by instantons. Pions are boundstates of quarks and the strong quark-pion vertex
is determined from an instanton induced effective lagrangian. The interaction
of the constituents of the pion with the external electromagnetic field is
introduced in gauge invariant form. The parameters of the model, i.e.,
effective instanton radius and constituent quark masses, are obtained from the
vacuum expectation values of the lowest dimensional quark and gluon operators
and the low-energy observables of the pion. We apply the formalism to the
calculation of the pion form factor by means of the isovector nonforward parton
distributions and find agreement with the experimental data.
