  We derive the quantum kinetic theory for fermions with arbitrary mass in a
background electromagnetic field from the Wigner-function approach. Since spin
of massive fermions is a dynamical degree of freedom, the kinetic equations
with the leading-order quantum corrections describe entangled dynamics of not
only the vector- and axial-charge distributions but also of the spin
polarization. Therefore, we obtain one scalar and one axial-vector kinetic
equations with magnetization currents pertinent to the spin-orbit interaction.
We show that our results smoothly reduce to the massless limit where the spin
of massless fermions is no longer an independent dynamical degree of freedom
but is enslaved by the chirality and momentum and the accordingly kinetic
equations turn into the chiral kinetic theory for Weyl fermions. We provide a
kinetic theory covering both the massive and massless cases, and hence resolves
the problem in constructing the bridge between them. Such generalization may be
crucial for applications to various physical systems. Based on our kinetic
equations, we discuss the anomalous currents transported by massive fermions in
thermal equilibrium.
