The structure of the proton is a fundamental and defining question for nuclear physics. The 12 GeV upgrade of Jefferson Lab, and likely construction of an Electron-Ion Collider in the coming decade, will render new aspects of this structure accessible to experiment. In particular, it can be anticipated that the three-dimensional internal structure of the proton will be mapped out in detail for the first time. This is encoded in the parton distribution functions that describe the longitudinal momentum distribution of quarks and gluons (partons) inside a fast moving hadron, and their extensions to generalized parton distributions (GPDs) and transverse momentum dependent parton distributions (TMDs) also describe the transverse position or momentum of the hadron's constituents. As co-spokesperson of the  TMD Topical Collaboration on transverse momentum dependent parton physics, I am working in complement to these experiments to develop a more complete theoretical understanding of the parton structure of hadrons and nuclei.  For example, my recent studies of complex aspects of the gluon structure of hadrons and nuclei, point the way towards obtaining QCD benchmarks and predictions for experiments at Jefferson Lab and the planned Electron-Ion Collider.