Protons and neutrons are known to be the building blocks of matter, and also known to be the bound states of quarks and gluons - the partons, whose dynamics is best described by Quantum Chromodynamics (QCD). Perturbative QCD has been very successful in interpreting and predicting high-energy scattering processes, in extracting the information on short-distance partonic dynamics, and in measuring the parton momentum distribution inside a nucleon. With the advance in experimental technique and precision, it became possible in recent years to probe parton's transverse motion and multiparton correlations inside a hadron. In this talk, I will discuss how to relate the parton's transverse motion to physical observables. I will then review recent theoretical advances in transverse momentum dependent (TMD) factorization for hard processes, since much of the predictive content of perturbative QCD is contained in factorization theorems. Finally, I will discuss the current effort and status in determining the TMD parton distributions and multiparton correlation functions.
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