Recent progress in the understanding of single spin asymmetries

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.