  We discuss a fundamental effect of the interaction-induced decoherence of the
electron wave function in disordered metals. In the first part of the paper we
consider a simple model of a quantum particle interacting with a bath of
harmonic oscillators and analyze the physical origin of the effect. This
exactly solvable model also allows to understand why the arguments against the
existence of the effect at low temperatures fail. The second part of the paper
is devoted to a rigorous analysis of quantum decoherence in disordered metals.
We also discuss the relation of our results to the recent experiments on GaAs
structures. The existence of a finite quantum decoherence rate at low $T$
implies that low dimensional disordered metals with generic parameters do not
become insulators even at T=0.
