  We present an effective field theory for a unified description of transport
in normal and superconducting metals in the presence of generic spin-orbit
coupling (SOC). The structure of the quantum kinetic theory in the diffusive
regime is determined by a set of fundamental constraints -- charge conjugation
symmetry, the causality principle, and the crystal symmetry of a material.
These symmetries uniquely fix the action of the Keldysh non-linear $\sigma$
model (NLSM), which at the saddle point yields the quantum kinetic Usadel-type
equation. Our phenomenological approach is reminiscent of the Ginzburg-Landau
theory, but is valid for superconductors in the whole temperature range,
describes the diffusive transport in the normal state, and naturally captures
the effects of superconducting fluctuations. As an application, we derive the
NLSM and quantum transport equations which include all effects of spin-orbit
coupling, allowed by the crystal symmetry, for example, the spin Hall, spin
current swapping or spin-galvanic effects. Our approach can be extended to
systems with broken time reversal symmetry, as well as to the description of
hybrid interfaces, where the spin-charge interconversion can be enhanced due to
strong interfacial SOC.
