Generalized TMDs in Hadronic Collisions

Generalized transverse momentum dependent parton distributions (GTMDs) are the most general parton correlation functions of hadrons. For a long time it was not clear if/how GTMDs can be measured. We argue that the exclusive double Drell-Yan process and $\eta_c$ production in hadronic collisions do give access to GTMDs. We also discuss how specific GTMDs can be measured by means of polarization observables.