Hadron structure is fully determined by equations of QCD, however, due to the low-energy behaviour of this theory is difficult to calculate from the first principles. The structure manifests itself in hard interactions involving hadrons in the initial or final state. In this context, it can be described in terms of parton distribution functions (PDFs). Three PDFs exist for each parton species. If the intrinsic transverse momentum of the partons cannot be neglected, the structure is even richer, with 8 transverse momentum dependent (TMD) PDFs at leading twist, describing correlations between the hadron and parton spin and the intrinsic transverse momentum. They were extensively studied by experiments at CERN, DESY, BNL, JLab and elsewhere and they form an important part of the future electron-ion collider (EIC). The experimental knowledge of these functions and the prospects of the near future will be reviewed.