TMDs are, in particular, sensitive to correlations between the transverse momentum of partons in the parent hadron and their spin or the hadron spin. In turn, the correlations provide access the dynamics of partons in the transverse plane in momentum space. Thus, TMDs are comparable and directly complementary to the generalized parton distributions (GPDs) which describe the parton dynamics in the transverse plane in position space. Formally, TMDs access the correlations between a parton orbital angular momentum (OAM) and the hadron/parton spin because they require wave function components with nonzero OAM. Therefore, TMDs allow us to study the full three-dimensional dynamics of hadrons, providing more detailed information than that contained in conventional PDF.