  A granular metal is an array of metallic nano-particles imbedded into an
insulating matrix. Tuning the intergranular coupling strength a granular system
can be transformed into either a good metal or an insulator and, in case of
superconducting particles, experience superconductor-insulator transition. The
ease of adjusting electronic properties of granular metals makes them most
suitable for fundamental studies of disordered solids and assures them a
fundamental role for nanotechnological applications. This Review discusses
recent important theoretical advances in the study of granular metals,
emphasizing on the interplay of disorder, quantum effects, fluctuations and
effects of confinement in formation of electronic transport and thermodynamic
properties of granular materials.
