  The Usadel equation is the standard theoretical tool for the description of
superconducting structures in the diffusive limit. Here I derive the Usadel
equation for gyrotropic materials with a generic linear in momentum spin-orbit
coupling. It accounts for the spin-charge/singlet-triplet coupling and in the
normal state reduces to the system of spin-charge diffusion equations
describing various magneto-electric effects, such as the spin Hall effect
(SHE), the spin-galvanic effect (SGE) and their inverses. Therefore the derived
Usadel equation establishes a direct connection of these effects to their
superconducting counterparts. The working power of the present formalism is
illustrated on the example of the bulk SGE.
