  The study of Whittaker models for representations of reductive groups over
local and global fields has become a central tool in representation theory and
the theory of automorphic forms. However, only generic representations have
Whittaker models. In order to encompass other representations, one attaches a
degenerate (or a generalized) Whittaker model $W_{\mathcal{O}}$, or a Fourier
coefficient in the global case, to any nilpotent orbit $\mathcal{O}$. In this
note we survey some classical and some recent work in this direction - for
Archimedean, p-adic and global fields. The main results concern the existence
of models. For a representation $\pi$, call the set of maximal orbits
$\mathcal{O}$ with $W_{\mathcal{O}}$ that includes $\pi$ the Whittaker support
of $\pi$. The two main questions discussed in this note are: (1) What kind of
orbits can appear in the Whittaker support of a representation? (2) How does
the Whittaker support of a given representation $\pi$ relate to other
invariants of $\pi$, such as its wave-front set?
