  This review article provides an overview of the physics of the two-channel
Kondo impurity model as manifested in two-level systems in metals and certain
actinide/lanthanide ions in metals. Basic models are presented, followed by a
discussion of the multiplicative renormalization group, numerical
renormalization group, non-crossing approximation, conformal field theory,
abelian bosonization, and Bethe-ansatz approaches to the models. An extensive
discussion of experiment is presented for both classes of models, with critical
attention paid to the relevance of the models to the data. Finally, a last
section details other models which are related, approaches to the lattice via
two impurity and infinite dimension models, and the connection of these models
to exotic odd-frequency superconductivity. Also available at
http://onsager.ucdavis.edu/~cox/REVIEW/drmpmain2.ps.gz
