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?
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