Electron-drag effects in coupled electron systems

The advancement of fabrication and lithography techniques of semiconductors have made it possible to study bi-layer systems made of two electronic layers separated by distances of several hundred Angstroms. In this situation the electrons in layer 1 are distinguishable from those in layer 2, and can communicate through the direct inter-layer Coulomb interaction. In particular, if a current is applied to one of the layers, the electrons in th e second will be dragged giving rise to a transresistance $\rho_D$. In this article we review recent theoretical and experimental developments in the understanding of this effect. At very low temperatures it turns out that phonons dominate the transresistance. The direct Coulomb interaction and plasmon excitations are important at temperatures $T>0.1T_F$, with $T_F$ the Fermi temperature. If a magnetic field is applied the transresistance is increased, in a very interesting interplay between $\rho_D$ and Landau quantization. The non-dissipative drag is also reviewed.