  The relevance of intrinsic (or primordial) transverse momentum of partons in
the inclusive production of particles at high energy and moderately large p_T
has been known for a long time, beginning with Drell-Yan and diphoton
processes, and continuing with photon and meson production in hadronic
collisions. In view of its renewed interest in the context of polarized
processes and single spin asymmetries we perform, in the framework of
perturbative QCD with the inclusion of spin and transverse momentum effects, a
detailed analysis of several such processes in different kinematical
situations. We show that the inclusion of these effects leads, at the level of
accuracy reachable in this approach, to an overall satisfactory agreement
between theoretical predictions and experimental unpolarized data, thus giving
support to the study of spin effects and single spin asymmetries within the
same scheme. We present results for transverse single spin asymmetries,
generated by the so-called Sivers effect, in inclusive pion and photon
production in proton-proton collisions. We compare our results with the
available experimental data and with previous results obtained using simplified
versions of this approach.
