  We study the electronic properties of an array of small metallic grains
connected by tunnel junctions. Such an array serves as a model for a granular
metal. Previous theoretical studies of junction arrays were based on models of
quantum dissipation which did not take into account the diffusive motion of
electrons within the grains. We demonstrate that these models break down at
sufficiently low temperatures: for a correct description of the screening
properties of a granular metal at low energies the diffusive nature of the
electronic motion within the grains is crucial. We present both a diagrammatic
and a functional integral approach to analyse the properties of junction
arrays. In particular, a new effective action is obtained which enables us to
describe the array at arbitrary temperature. In the low temperature limit, our
theory yields the correct, dynamically screened Coulomb interaction of a normal
metal, whereas at high temperatures the standard description in terms of
quantum dissipation is recovered.
