Transverse Single-Spin Asymmetries: Are They Understood?

Transverse single-spin asymmetries (SSAs) are intriguing observables that have been measured in several hard scattering processes. Their description in the framework of Quantum Chromodynamics (QCD), the generally accepted theory of the strong interaction, is challenging. In order to deal with such effects one needs to go beyond the frequently used collinear parton approximation and/or take multi-parton correlations into account. It is discussed to what extent transverse SSAs are understood in QCD. We will also address questions like what SSAs teach us about the structure of the nucleon in three dimensions, and how they allow us to test QCD factorization.