Exclusive electroproduction and off-diagonal parton distributions

7 pages, 3 figures, LaTeX2e. Talk given at the Fourth Workshop on Quantum Chromodynamics, Paris, France, 1-6 June 1998International audienceOff-diagonal parton distributions occur in several hard exclusive reactions. They extend the study of hadron structure beyond what can be learned from ordinary distributions and have a particularly rich spin structure. The hard scattering subprocesses in electroproduction of mesons and of real photons satisfy helicity selection rules, which provide powerful tools to test leading-twist dominance at a given value of the hard scale