  Applications of perturbative QCD to deeply virtual Compton scattering and
hard exclusive electroproduction processes require a generalization of usual
parton distributions for the case when long-distance information is accumulated
in nonforward matrix elements of quark and gluon light-cone operators. We
describe two types of nonperturbative functions parametrizing such matrix
elements: double distributions F(x,y;t) and skewed distribution functions
  F_\zeta(X;t), discuss their properties, and basic uses in the QCD description
of hard exclusive processes.
