We describe the variable flavor number scheme to three-loop order, which modifies
the massless parton densities by single- and two-mass effects and introduces heavy-quark
parton distribution functions for charm and bottom. A renormalization group analysis
shows the validity of this picture at large scales $Q^2$, where it resembles the
non-power-suppressed heavy-flavor corrections completely. We also provide numerical
implementations of a series of charged and neutral current Wilson coefficients.