  One of the most fundamental limitations of a muon-spin relaxation experiment
can be the lack of knowledge of the implantation site of the muon and the
uncertainty about the muon's perturbation of its host. Here we review some of
the work done on the `muon site problem' in the solid state and highlight some
recent applications of electronic structure calculations that have successfully
characterized the quantum states of muons in a number of insulating compounds
containing fluorine, in a number of pnictide superconductors, and in ZnO.
