In this review we discuss the phenomenon of counterrotation, observed in ~40 apparently normal galaxies where gas or stars are rotating in a direction opposite to the main stellar body. It appears in all morphological types, from ellipticals to spirals or irregulars.
The problem of the origin and the evolution of counterrotation is discussed, in the wider frame of the galaxies accreting matter from their environment (e.g., minor-axis dust-lane ellipticals or polar-ring S0s). Various sources of matter appear to be candidates for the production of the counterrotation.
Due to the tendency of the bar potential to populate particular families of orbits, some cases of counterrotation found in barred galaxies are interpreted as intrinsic to barred galaxies.