  The term quantum turbulence denotes the turbulent motion of quantum fluids,
systems such as superfluid helium and atomic Bose-Einstein condensates which
are characterized by quantized vorticity, uperfluidity and, at finite
temperatures, two-fluid behavior. This article introduces their basic
properties, describes types and regimes of turbulence which have been observed,
and highlights similarities and differences between quantum turbulence and
classical turbulence in ordinary fluids. Our aim is also to link together the
articles of this special issue, and to provide a perspective of the future
development of a subject which contains aspects of fluid mechanics, atomic
physics, condensed matter and low temperature physics.
