Understanding the nucleon structure is of fundamental importance to science. The exploration of the internal structure of the nucleon in terms of quarks and gluons (partons), the degrees of freedom of Quantum Chromodynamics (QCD), has been and still is at the frontier of high energy nuclear physics research. Concurrent advances in the experimental use of high energy scattering processes and theoretical breakthroughs in understanding "asymptotic freedom" and developing the perturbation theory of strong interactions have provided a way of mapping out the internal landscape of protons and neutrons. In past decades, a one-dimensional picture of nucleons has emerged, in the sense that we "only" learn about the "longitudinal" motion of partons in fast moving protons and neutrons, as encoded in the so-called collinear parton distribution functions (PDFs). An important and yet unresolved question is what role the parton's transverse motion may play. To describe the partons in the transverse plane in momentum space, one introduces the so-called Transverse Momentum Dependent PDFs (TMDs). TMDs provide new and much richer information on the nucleon structure: they allow for the first time to carry out 3D-imaging of the nucleon. In this talk, I will present to you these new QCD frontiers in theory along with their exciting experimental programs, and demonstrate to you how they have opened new windows to study QCD dynamics and nucleon structure. In the end I will also discuss the future opportunities in such a rapidly developing field.
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