The Rayleigh Taylor (RT) instability is a universal hydrodynamic process, which transforms excess of potential energy into kinetic energy of the flow. The instability develops when a heavy gas (fluid, plasma) is supported by a light one in a gravitational field, real or effective.
 In this talk I will consider development of the RT instability at the linear and nonlinear stages in applications to inertial confinement fusion, flames, quantum gases and thermonuclear Supernovae. In inertial confinement fusion, we will discuss linear stabilization
of the RT instability by the laser ablation flow, and dynamics of the heated plasma bubbles at the nonlinear instability stage. The RT instability in the ablation flow may be also accompanied by the Darrieus-Landau instability of the laser deflagration front.
In the context of combustion science, the RT instability is traditionally encountered for upward propagating flames. Another important example is provided by fast burning along the vortex axes, which may be quite important for turbulent combustion. In thermonuclear Supernovae, the RT instability results in formation of large scale bubbles of light burnt matter, which move the thermonuclear burning process from the star center to the outer layers, thus preventing from a Supernova explosion in a purely
deflagration regime.