Rayleigh-Taylor (RT) instability at the pusher-fuel interface in inertial confinement fusion (ICF) targets may significantly degrade the thermonuclear burn. Direct three dimensional numerical simulation and experiments are used to investigate turbulent mixing due to Rayleigh-Taylor instability in simple situations. The results obtained help to construct a two-dimensional turbulence model which may be used to assess the combined effect of implosion asymmetry and small-scale turbulent mixing in ICF targets.