  Parton distribution functions are key quantities for us to understand the
hadronic structures in high-energy scattering, but they are difficult to
calculate from lattice QCD. Recent years have seen fast development of the
large-momentum effective theory which allows extraction of the $x$-dependence
of parton distribution functions from a quasi-parton distribution function that
can be directly calculated on lattice. The extraction is based on a
factorization formula for the quasi-parton distribution function that has been
derived rigorously in perturbation theory. A systematic procedure that includes
renormalization, perturbative matching, and power corrections has been
established to calculate parton distribution functions. Latest progress from
lattice QCD has shown promising signs that it will become an effective tool for
calculating parton physics.
