  Tunneling of single electrons has been thoroughly studied both theoretically
and experimentally during last ten years. By the present time the basic physics
is well understood, and creation of useful single-electron devices becomes the
important issue. Single-electron tunneling seems to be the most promising
candidate to be used in the future integrated digital circuits with the typical
size scale of few nanometers and below, i.e. in the molecular electronics. In
the review we first briefly discuss the physics of single-electron tunneling
and the operation of the single-electron transistor. After that, we concentrate
on the hypothetical ultradense digital single-electron circuits and discuss the
different proposed families of them. The last part of the review considers the
issues of the discrete energy spectrum and the finite tunnel barrier height
which are important for the molecular-size single-electron devices.

Condensed Matter