The modern formulation of exclusive reactions within Quantum Chromodynamics
is reviewed, the emphasis being placed on the pivotal ideas and methods
pertaining to perturbative and non-perturbative topics. Specific problems,
related to scale locality, infrared safety, gluonic radiative corrections
(Sudakov effects), and the role of hadronic size effects (intrinsic transverse
momentum), are studied. These issues are more precisely analyzed in terms of
the essential mechanisms of momentum transfer to a hadron while remaining
intact. Different factorization schemes are considered and the conceptual
lacunas are pointed out. The quite technical subject of renormalization-group
evolution is given a detailed account. By combining analytical and numerical
algorithms, the one-gluon exchange nucleon evolution equation is diagonalized
and next-to-leading eigenfunctions are calculated in terms of Appell
polynomials. The corresponding anomalous dimensions of trilinear quark
operators are found to form a degenerate system whose envelope shows
logarithmic large-order behavior. Selected applications of this framework are
presented, focusing on the helicity-conserving elastic form factors of the pion
and the nucleon. The theoretical constraints imposed by QCD sum rules on the
moments of nucleon distribution amplitudes are used to determine a whole
spectrum of optional solutions. They organize themselves along an ``orbit''
characterized by a striking scaling relation between the form-factor ratio
R=β£GMnββ£/GMpβ and the projection coefficient
B4β on to the corresponding eigensolution. The main reasons for the failure
of the present theoretical predictions to match the experimental data are
discussed and workable explanations are sketched.Comment: 112 pages; 12 tables; 35 embedded figures as PS/EPS files; RevTex
styles used. Article based on thesis for Dr. nauk (sci.) phys.-math. degree,
successfully defended at BLThP, JINR, Dubna (1997); Bochum Reports
RUB-TPII-20/96, RUB-TPII-07/99. Published in Eur. Phys. J. direct C7, 1-109
(1999