  We describe a broadly-applicable theory of spin relaxation in materials with
incoherent charge transport; examples include amorphous inorganic
semiconductors, organic semiconductors, quantum dot arrays, and systems
displaying trap-controlled transport or transport within an impurity band. The
theory can incorporate many different relaxation mechanisms, so long as
electron-electron correlations can be neglected. We focus primarily on spin
relaxation caused by spin-orbit effects, which manifest through inhomogeneities
in the $g$-factor and non-spin-conserving carrier hops, scattering, trapping,
or detrapping. Analytic and numerical results from the theory are compared in
various regimes with Monte Carlo simulations. Our results should assist in
evaluating the suitability of various disordered materials for spintronic
devices.
