  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.
