  We describe two complementary formalisms designed for the description of
probability density function (PDF) of the gradients of turbulent fields. The
first approach, we call it adiabatic, describes PDF at the values much less
than dispersion. The second, instanton, approach gives the tails of PDF at the
values of the gradient much larger than dispersion. Together, both approaches
give satisfactory description of gradient PDFs, as illustrated here by an
example of a passive scalar advected by a one-dimensional compressible random
flow.
