  We consider the transverse-momentum (q_T) distribution of generic high-mass
systems (lepton pairs, vector bosons, Higgs particles, ....) produced in hadron
collisions. At small q_T, we concentrate on the all-order resummation of the
logarithmically-enhanced contributions in QCD perturbation theory. We elaborate
on the $b$-space resummation formalism and introduce some novel features: the
large logarithmic contributions are systematically exponentiated in a
process-independent form and, after integration over q_T, they are constrained
by perturbative unitarity to give a vanishing contribution to the total cross
section. At intermediate and large q_T, resummation is consistently combined
with fixed-order perturbative results, to obtain predictions with uniform
theoretical accuracy over the entire range of transverse momenta. The formalism
is applied to Standard Model Higgs boson production at LHC energies. We combine
the most advanced perturbative information available at present for this
process: resummation up to next-to-next-to-leading logarithmic accuracy and
fixed-order perturbation theory up to next-to-leading order. The results show a
high stability with respect to perturbative QCD uncertainties.
