  We review the recently discovered spin Hall magnetoresistance (SMR) and
related effects from a theoretical point of view. The SMR is observed in
bilayers of a magnetic insulator and a metal, in which spin currents
aregenerated in the normal metal due to the spin Hall effect. The associated
angular momentum transfer to the ferromagnetic layer and thereby the electrical
resistance is modulated by the angle between the applied current and the
magnetization direction. The SMR provides a convenient tool to non-invasively
measure the magnetization direction and spin-transfer torque to an insulator.
We introduce the minimal theoretical instruments to calculate the SMR, i.e.
spin diffusion theory and quantum mechanical boundary conditions. This leads to
a small set of parameters that can be fitted to experiments. We discuss the
limitations of the theory as well as alternative mechanisms such as the
ferromagnetic proximity effect and Rashba spin-orbit torques, and point out new
developments related to the SMR.
