  Over the past years a lot of progress has been made in the understanding of
single spin asymmetries in hard scattering processes. We briefly review this
subject, covering the non-vanishing of time-reversal odd parton distributions,
universality of fragmentation functions, and the discovery of previously
unknown time-reversal odd parton distributions at subleading twist.
Interestingly enough, all these developments were initiated by simple model
calculations.
