Dispersivity (patchiness, filamentary structure, and so forth) in astrophysical
objects is common; this dispersivity is associated with turbulence, which is
partially the result of hydrodynamic instability. In many cases, “exchange” or
“interchange” turbulence associated with unstable stratification in a gravita-
tional field and bouyant forces is important. The corresponding instability is
called Rayleigh-Taylor, exchange, interchange, or sometimes gravitational
instability; it is sometimes described in terms of the appearance of modes that
arise, for example, in the case of flute instability, sausage instability, etc. In
other cases, Richtmyer—Meshkov instability occurs; this is brought about by
initial velocity fluctuations. This review presents the most up-to-date informa-
tion in this field of interest to astrophysicists, collected from various branches
of science, and describes a number of astrophysical applications.