  The phenomenon of counterrotation is observed when two galaxy components have
their angular momenta projected antiparallel onto the sky. It follows that if
the two components rotate around the same axis, the counterrotation is
intrinsic. On the contrary the counterrotation is only apparent if the rotation
axes are misaligned and the line-of-sight lies in between the two vectors or
their antivectors. In the case of intrinsic counterrotation the two components
can be superimposed or radially separated. As far as the two components are
concerned, stars are observed to counterrotate with respect to other stars or
gas. The counterrotation of gas versus gas has been also detected. Up to now,
the number of galaxies exhibiting these phenomena are $\sim60$, the
morphological type of which ranges from ellipticals to S0's and to spirals.
  When a second event occurs in a galaxy, such as the acquisition of material
from outside, it is likely that the resulting angular momentum of the acquired
material is decoupled from the angular momentum of the preexisting galaxy.
Counterrotation is therefore a general signature of material acquired from
outside the main confines of the galaxy. Good examples of such cases are
ellipticals with a dust lane or gaseous disk along the minor axis and polar
ring galaxies, where the angular momenta are perpendicular. It should be noted
that recently attempts have been made to explain special cases of stars versus
stars counterrotation in disk galaxies as due to a self induced phenomenon in
non-axisymmetric potentials. We discuss the phenomenon according to the
morphological type and to the kind of counterrotation.
