  We present an overview on the current experimental and phenomenological
status of transverse single spin asymmetries (tSSAs) in proton-proton
collisions. In particular, we focus on large-$p_T$ inclusive pion, photon, jet,
pion-jet production and Drell-Yan processes. For all of them theoretical
estimates are given in terms of a generalised parton model (GPM) based on a
transverse momentum dependent (TMD) factorisation scheme. Comparisons with the
corresponding results in a collinear twist-3 formalism and in a modified GPM
approach are also made. On the experimental side, a selection of the most
interesting and recent results from RHIC is presented.
