Theory overview on the TMD physics and its role in understanding nucleon structure

Transverse-momentum-dependent parton distribution functions (or simply, TMDs) are fundamental quantum correlation functions of QCD, providing unique insight into the parton momentum and spin structure of hadrons, as well as information on momentum-spin correlations. In this talk, I will give a comprehensive theory overview of TMDs and their connections to physical observables in terms of QCD factorization. I will also discuss the challenges for extracting TMDs phenomenologically from experimental data and lattice QCD simulations, focusing on consistent and systematic ways to disentangle collision-specific and intrinsic and process-independent contributions to the observed transverse momentum dependence of TMDs and their roles in describing the confined partonic structure of hadrons.