  The article reviews recent results for the low energy physics of fast
tunneling centers in metallic environments. For strong enough couplings to the
environment these tunneling centers display an orbital Kondo effect and give
rise to a non-Fermi-liquid behavior. This latter property is explained by
establishing a mapping of the tunneling center model to the multichannel Kondo
model via the renormalization group transformation combined with a $1/N_f$
expansion. The case of M-state systems, the role of the splittings and the
present experimental situation are also discussed.
