  Superfluid turbulence consisting of quantized vortices is called quantum
turbulence (QT). Quantum turbulence and quantized vortices were discovered in
superfluid $^4$He about 50 years ago, but innovation has occurred recently in
this field. One is in the field of superfluid helium. Statistical quantities
such as energy spectra and probability distribution function of the velocity
field have been accessible both experimentally and numerically. Visualization
technique has developed and succeeded in the direct visualization of quantized
vortices. The other innovation is in the field of atomic Bose-Einstein
condensation. The modern optical technique has enabled us to control and
visualize directly the condensate and quantized vortices. Various kinds of
hydrodynamic instability have been revealed. Even QT is realized
experimentally. This article describes such recent developments as well as the
motivation of studying QT.
