  We derive Boltzmann equations for massive spin-1/2 fermions with local and
nonlocal collision terms from the Kadanoff--Baym equation in the
Schwinger--Keldysh formalism, properly accounting for the spin degrees of
freedom. The Boltzmann equations are expressed in terms of matrix-valued spin
distribution functions, which are the building blocks for the quasi-classical
parts of the Wigner functions. Nonlocal collision terms appear at
next-to-leading order in $\hbar$ and are sources for the polarization part of
the matrix-valued spin distribution functions. The Boltzmann equations for the
matrix-valued spin distribution functions pave the way for simulating
spin-transport processes involving spin-vorticity couplings from first
principles.
