  We present a pedagogical account of our approach to the problem of Luttinger
liquids coupled by interliquid single particle hopping. It is shown that the
key issue is that of coherence/incoherence of interliquid hopping, and not of
relevance/irrelevance in a renormalization group sense. A clear signal of
coherence, present in the case of coupled Fermi liquids, is absent for
Luttinger liquids, and we argue for the existence of an incoherent regime when
the interliquid hopping rate is sufficiently small. The problem is relevant to
any sufficiently anisotropic, strongly correlated metal, and in particular to
understanding the anomalous c-axis conductivity in the cuprate superconductors,
and the physics of the quasi-1D organic conductors.
